LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


Class 


ELEMENTARY  METEOROLOGY 


FOR  HIGH  SCHOOLS  AND   COLLEGES 


BY 


FRANK   WALDO,    PH.D. 

LATK  JUNIOR  PROFESSOR  IN  THE  UNITED  STATES  SIGNAL  SERVICE,  MEMBER  OF 

THE  AUSTRIAN  AND  GERMAN  METEOROLOGICAL  SOCIETIES, 

AUTHOR  OF  "MODERN  METEOROLOGY,"  ETC. 


NEW  YORK-:- CINCINNATI-:.  CHIC  AGO 

AMERICAN    BOOK    COMPANY 


COPYRIGHT,  1896,  BY 
AMERICAN    BOOK  COMPANY 

WALDO'S  METEOR. 


PREFACE. 

METEOROLOGY,  which  treats  of  our  atmosphere,  is  a  department 
of  the  physics  of  the  globe,  or  "  geo-physics,"  as  it  is  called.  It  is 
only  within  recent  years  that  meteorology  has  been  elevated  to  the 
position  of  an  independent  science. 

The  more  apparent  atmospheric  conditions  have  been  the  sub- 
ject of  observation  and  comment  for  many  hundreds  of  years,  but 
only  within  the  past  two  or  three  centuries  have  accurate  observa- 
tions and  trustworthy  records  been  accumulated.  These  observa- 
tions, which  have  greatly  increased  in  number  and  accuracy  during 
the  present  century,  and  especially  during  the  past  twenty  years, 
are  the  groundwork  which  serves^  as  a  basis  for  the  inductive 
development  of  the  science  of  meteorology ;  and  they  also  serve 
as  a  criterion  for  the  testing^  o'f:  the  truths  evolved  by  the  deduc- 
tive method  which  has  been  developed  by  mathematical  physicists 
almost  entirely  within  the  last  quarter  of  a  century. 

It  has  been  the  custom  to  offer  to  English-reading  students  of 
meteorology  the  development  of  the  subject  from  the  inductive 
standpoint ;  and  in  this  elementary  treatise  the  same  form  has 
been  in  the  main  adopted. 

In  the  deductive  treatment  it  would  be  necessary  to  start  with 
initial  conditions,  —  density,  temperature,  moisture  content,  and 
motion  of  the  air,  —  and  then  follow  out  the  regular  and  the  irreg- 
ular changes  of  possible  occurrence,  keeping  in  mind  the  relation 
between  all  of  these  conditions  and  the  effect  of  one  on  another. 
Such  a  treatment,  while  certainly  more  logical,  and  on  this  account 
more  desirable,  is  vastly  more  difficult,  than  the  method  adopted, 
and  should  be  offered  to  mature  minds  only,  which  can  grasp  very 
complex  processes. 

It  was  therefore  decided  to  give  merely  the  facts  and  their 
probable  explanations,  in  treating  the  subject  of  the  atmospheric 
conditions  ;  furthermore,  the  elements  have  as  far  as  possible  been 
mentioned  separately,  in  order  to  avoid  the  confusion  of  a  more 
complex  treatment,  and  the  better  to  isolate  the  obscure  and 
uncertain  parts  of  the  subject. 

In  treating  of  the  atmospheric  movements,  the  author  has,  how- 
ever, combined  the  two  methods  of  presentation. 

3 


.(.  PREFACE. 

The  best  example  that  we  have  of  a  non-mathematical  deductive 
treatment  of  meteorology  is  Ferrel's  "  Popular  Treatise  on  the 
Winds,"  in  which  that  author  has  given  the  most  complete  account 
of  the  atmospheric  circulation  and  allied  phenomena  to  be  found 
in  any  language,  and  all  teachers  and  advanced  students  of  meteor- 
ology are  earnestly  recommended  to  read  Ferrel's  book. 

In  order  to  keep  this  book  within  proper  limits,  the  author  has 
been  obliged  to  omit  much  matter  which  should  find  place  in  a 
complete  treatment  of  the  subject.  Thus  but  the  barest  mention 
could  be  made  of  the  simpler  forms  of  meteorological  apparatus ; 
and  the  application  of  meteorology  to  the  sciences  and  arts,  such 
as  hygiene,  agriculture,  engineering,  and  to  manufacturing  and 
commerce,  has  been  entirely  excluded. 

The  scope  of  this  book  must  not,  therefore,  be  misinterpreted 
by  meteorologists  on  the  one  hand,  or  by  teachers  on  the  other. 
It  is  intended  to  serve  as  a  text-book  of  the  elements  of  the  science 
for  general  students,  and  must  not  be  considered  as  a  manual  for 
practicing  meteorologists. 

Quantitative  results  have  been  given  as  far  as  possible  in  round 
numbers.  The  English  systems  of  measurements  are  given,  as 
necessary  in  a  book  intended  for  immediate  and  widespread  use 
in  our  educational  institutions. 

In  the  preparation  of  this  book  the  author  has  drawn  on  his 
experience  as  a  former  teacher  of  meteorology  in  the  school  of 
training  for  officers  and  observers  of  the  Weather  Bureau  of  the 
United  States  Signal  Service  ;  and  he  has  tested  much  of  the  sub- 
ject-matter herein  presented  (although  in  somewhat  greater  detail) 
in  a  course  of  lectures  delivered  at  Evelyn  College  for  Young 
Women,  at  Princeton. 

The  author  is  much  indebted  to  Lieutenant  Everett  Hayden, 
[J.S.N.,  for  suggestions  and  assistance  in  reading  the  final  proof 
sheets. 

Many  of  the  diagrams  in  the  book  have  been  redrawn  from 
other  printed  sources,  both  foreign  and  American  :  among  the 
latter  must  be  mentioned  in  particular  Ferrel's  "  Popular  Treatise 
on  the  Winds  "  and  Greely's  "American  Weather." 

FKANK    WALDO. 
PKINCKTON,  X.J. 


CONTENTS. 


[.    THE  EARTH'S  ATMOSPHERE 


II.    TEMPERATURE 19 

Heat  and  Solar  Radiation  .         .         .  .         .         .19 

Thennometry        .         . 31 

Observed  Air  Temperatures          ......       34 

Distribution  of  Air  Temperatures  over  the  Earth       .         .       50 
Temperatures  below  the  Earth's  Surface     ....       70 

III.  AIR  PRESSURE 73 

Barometry    ..........       75 

Observed  Air  Pressures        .......       80 

Distribution  of  Air  Pressures  over  the  Earth      ...       88 

IV.  WINDS ioi 

Classification  of  Winds         .......     102 

Observations  of  Wind  Direction 103 

Observations  of  Wind  Velocity    ......     106 

V.     MOISTURE:  VAPOR,  CLOUD 118 

Atmospheric  Moisture.         .         .  .  .  .  .  .118 

Atmospheric  Moisture  as  Vapor  .  .  .  .  .122 

Observed  Atmospheric  Humidity  .  .  .  .  .124 

Atmospheric  Moisture  as  Cloud  and  Fog  .  .  .  .129 

Clouds.         .         .         .         .         .  .  .  .  .  -130 

Observations  of  Cloudiness .         .  .  .  .  .  .     137 

5 


6  CONTENTS. 

CHAPTER  PAGE 

VI.     MOISTURE:  PRECIPITATION 142 

Condensation        .         .         . 143 

Observations  of  Rainfall 145 

Distribution  of  Rainfall  over  the  Earth       ....  148 

Hail  and  Snow .         .         .159 

Evaporation -163 

VII.    ATMOSPHERIC  OPTICS  AND  ELECTRICITY 166 

Atmospheric  Optics 166 

Atmospheric  Electricity 175 

VIII.     GENERAL  CIRCULATION  OF  THE  ATMOSPHERE    ....  180 

General  Air  Motions    . 180 

Primary  Circulation  of  the  Atmosphere       ....  187 

IX.     SECONDARY  CIRCULATION  OK  THE  ATMOSPHERE         .         .        .  213 
Cyclones       .         .         .         .         .         .         .         .         .         .216 

Anticyclones         ..........  234 

X.     LOCAL  AND  MISCELLANEOUS  'WINDS 241 

Tornadoes    ..........  241 

Thunderstorms      .........  249 

Spouts ........         .         .         .  259 

Periodic  Local  Winds 262 

Miscellaneous  Winds   ........  263 

XI.     WEATHER  AND  WEAIIIER  PREDICTIONS 269 

Weather  Conditions .  269 

Weather  Predictions -274 

XII.     CLIMATE 293 

Climatic  Conditions      ........  293 

Climates  of  the  Continents.          ......  307 

XIII.     CLIMATE  OF  THE  UNITED  STATES      ......  313 

Climatic  Subdivisions  of  the  United  States    .         .         .         .  317 

Distribution  of  the  Climatological  Elements  over  the  United 
States     

INDEX 


ELEMENTARY  METEOROLOGY 


FOR  HIGH  SCHOOLS  AND  COLLEGES 


BY 


FRANK   WALDO,    PH.D. 

LATE  JUNIOR  PROFESSOR  IN  THE  UNITED  STATES  SIGNAL  SERVICE,  MEMBER  or 

THE  AUSTRIAN  AND  GERMAN   METEOROLOGICAL  SOCIETIES, 

AUTHOR  OF  "  MODERN  METEOROLOGY,"  BTC. 


OF   THE 

UNIVERSITY 

OF 


NEW  YORK- :-CINCINN ATI- :•  CHICAGO 

AMERICAN    BOOK    COMPANY 


COPYRIGHT,  1896,  BY 
AMERICAN   BOOK  COMPANY 

WALDO'S  METEOR. 


PREFACE. 

METEOROLOGY,  which  treats  of  our  atmosphere,  is  a  department 
of  the  physics  of  the  globe,  or  "  geo-physics,"  as  it  is  called.  It  is 
only  within  recent  years  that  meteorology  has  been  elevated  to  the 
position  of  an  independent  science. 

The  more  apparent  atmospheric  conditions  have  been  the  sub- 
ject of  observation  and  comment  for  many  hundreds  of  years,  but 
only  within  the  past  two  or  three  centuries  have  accurate  observa- 
tions and  trustworthy  records  been  accumulated.  These  observa- 
tions, which  have  greatly  increased  in  number  and  accuracy  during 
the  present  century,  and  especially  during  the  past  twenty  years, 
are  the  groundwork  which  serves  as  a  basis  for  the  inductive 
development  of  the  science  of  meteorology ;  and  they  also  serve 
as  a  criterion  for  the  testing  of  the  truths  evolved  by  the  deduc- 
tive method  which  has  been  developed  by  mathematical  physicists 
almost  entirely  within  the  last  quarter  of  a  century. 

It  has  been  the  custom  to  offer  to  English-reading  students  of 
meteorology  the  development  of  the  subject  from  the  inductive 
standpoint ;  and  in  this  elementary  treatise  the  same  form  has 
been  in  the  main  adopted. 

In  the  deductive  treatment  it  would  be  necessary  to  start  with 
initial  conditions,  —  density,  temperature,  moisture  content,  and 
motion  of  the  air,  —  and  then  follow  out  the  regular  and  the  irreg- 
ular changes  of  possible  occurrence,  keeping  in  mind  the  relation 
between  all  of  these  conditions  and  the  effect  of  one  on  another. 
Such  a  treatment,  while  certainly  more  logical,  and  on  this  account 
more  desirable,  is  vastly  more  difficult,  than  the  method  adopted, 
and  should  be  offered  to  mature  minds  only,  which  can  grasp  very 
complex  processes. 

It  was  therefore  decided  to  give  merely  the  facts  and  their 
probable  explanations,  in  treating  the  subject  of  the  atmospheric 
conditions  ;  furthermore,  the  elements  have  as  far  as  possible  been 
mentioned  separately,  in  order  to  avoid  the  confusion  of  a  more 
complex  treatment,  and  the  better  to  isolate  the  obscure  and 
uncertain  parts  of  the  subject. 

In  treating  of  the  atmospheric  movements,  the  author  has,  how- 
ever, combined  the  two  methods  of  presentation. 

3 


4  PREFACE. 

The  best  example  that  we  have  of  a  non-mathematical  deductive 
treatment  of  meteorology  is  Ferrel's  "  Popular  Treatise  on  the 
Winds,"  in  which  that  author  has  given  the  most  complete  account 
of  the  atmospheric  circulation  and  allied  phenomena  to  be  found 
in  any  language,  and  all  teachers  and  advanced  students  of  meteor- 
ology are  earnestly  recommended  to  read  Ferrel's  book. 

In  order  to  keep  this  book  within  proper  limits,  the  author  has 
been  obliged  to  omit  much  matter  which  should  find  place  in  a 
complete  treatment  of  the  subject.  Thus  but  the  barest  mention 
could  be  made  of  the  simpler  forms  of  meteorological  apparatus ; 
and  the  application  of  meteorology  to  the  sciences  and  arts,  such 
as  hygiene,  agriculture,  engineering,  and  to  manufacturing  and 
commerce,  has  been  entirely  excluded. 

The  scope  of  this  book  must  not,  therefore,  be  misinterpreted 
by  meteorologists  on  the  one  hand,  or  by  teachers  on  the  other. 
It  is  intended  to  serve  as  a  text-book  of  the  elements  of  the  science 
for  general  students,  and  must  not  be  considered  as  a  manual  for 
practicing  meteorologists. 

Quantitative  results  have  been  given  as  far  as  possible  in  round 
numbers.  The  English  systems  of  measurements  are  given,  as 
necessary  in  a  book  intended  for  immediate  and  widespread  use 
in  our  educational  institutions. 

In  the  preparation  of  this  book  the  author  has  drawn  on  his 
experience  as  a  former  teacher  of  meteorology  in  the  school  of 
training  for  officers  and  observers  of  the  meteorological  depart- 
ment of  the  United  States  Signal  Service ;  and  he  has  tested  much 
of  the  subject-matter  herein  presented  (although  in  somewhat 
greater  detail)  in  a  course  of  lectures  delivered  at  Evelyn  College 
for  Young  Women,  at  Princeton. 

The  author  is  much  indebted  to  Lieutenant  Everett  Hayden, 
U.S.N.,  for  suggestions  and  assistance  in  reading  the  final  proof 
sheets. 

Many  of  the  diagrams  in  the  book  have  been  redrawn  from 
other  printed  sources,  both  foreign  and  American  :  among  the 
latter  must  be  mentioned  in  particular  Ferrel's  "  Popular  Treatise 
on  the  Winds  "  and  Greely's  "American  Weather." 

FRANK   WALDO. 
PRINCETON,  N.J. 


CONTENTS. 


CHAPTER  PAGE 

I.    THE  EARTH'S  ATMOSPHERE        .        .        .        0        0        ..        .  „    -7 

II.    TEMPERATURE .        .  .19 

Heat  and  Solar  Radiation  19 

Thermometry .31 

Observed  Air  Temperatures         .         .                  .         .  -34 

Distribution  of  Air  Temperatures  over  the  Earth       .  .       50 

Temperatures  below  the  Earth's  Surface     .         .         .  -7° 

III.  AIR  PRESSURE -73 

Barometry    . -75 

Observed  Air  Pressures        .  80 

Distribution  of  Air  Pressures  over  the  Earth      ...       88 

IV.  WINDS 101 

Classification  of  Winds 102 

Observations  of  Wind  Direction 103 

Observations  of  Wind  Velocity   .         .         .         .         .         .  106 

V.    MOISTURE:  VAPOR,  CLOUD 118 

Atmospheric  Moisture 118 

Atmospheric  Moisture  as  Vapor 122 

Observed  Atmospheric  Humidity 124 

Atmospheric  Moisture  as  Cloud  and  Fog  .         .         .         .129 

Clouds.         ......                  ...  130 

Observations  of  Cloudiness .          .         .         .         .         .         .  137 

5 


6  CONTENTS. 

CHAPTER  PAGE 

VI.    MOISTURE:  PRECIPITATION         ....«,,     142 
Condensation        ..         ...»         ...     143 

Observations  of  Rainfall      .         .         .         .         ,         „         .145 
Distribution  of  Rainfall  over  the  Earth      .  148 

Hail  and  Snow 159 

Evaporation          .         .         .         .         „         .         „         0         .163 

VII.    ATMOSPHERIC  OPTICS  AND  ELECTRICITY 166 

Atmospheric  Optics 166 

Atmospheric  Electricity        .         .         .         .         .         .         -175 

VIII.     GENERAL  CIRCULATION  OF  THE  ATMOSPHERE    .        .        .        .180 

General  Air  Motions „         .180 

Primary  Circulation  of  the  Atmosphere       .         .         .         .187 

IX.    SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE        „        .        .    213 

Cyclones' „     216 

Anticyclones 234 

X.    LOCAL  AND  MISCELLANEOUS  WINDS 241 

Tornadoes 241 

Thunderstorms 249 

Spouts ....  259 

Periodic  Local  Winds 262 

Miscellaneous  Winds   ........  263 

XI.     WEATHER  AND  WEATHER  PREDICTIONS 269 

Weather  Conditions     ........     269 

Weather  Predictions     ........     274 

XII.    CLIMATE ....    293 

Climatic  Conditions      .         .         .                  .  293 

Climates  of  the  Continents .     307 

XIII.    CLIMATE  OF  THE  UNITED  STATES 313 

Climatic  Subdivisions  of  the  United  States    .         .         .         -317 
Distribution  of  the  Climatological  Elements  over  the  United 

States 321 

INDEX .    365 


\ 

•      \ 


ELEMENTARY  METEOROLOGY, 


CHAPTER   I. 
THE  EARTH'S   ATMOSPHERE. 

Meteorology,  strictly  speaking,  is  the  science  which 
treats  of  the  condition  of  the  atmosphere,  its  changes  in 
condition,  and  the  causes  which  give  rise  to  these  condi- 
tions and  changes.  The  science  is  as  yet  but  partially 
developed,  and  much  that  is  at  present  accepted  as  fact 
will  be  modified  by  future  investigations. 

Air.  —  The  gaseous  envelope  which  immediately  sur- 
rounds the  earth,  and  is  called  the  atmosphere,  is  com- 
posed chiefly  of  air,  just  as  the  ocean  is  composed  of 
water.  Air  is  a  mechanical  mixture  of  oxygen  and  nitro- 
gen gases  and  a  small  amount  of  carbon  dioxide,  together 
with  argon  and  traces  of  various  other  chemical  substances 
which  have  little  to  do  with  the  science  of  meteorology 
as  now  studied.  Such  air  is  called  dry  air. 

Atmospheric  air  always  contains  more  or  less  vapor  of 
water,  which  varies  in  amount  from  a  very  small  quantity 
to  about  5%  of  the  amount  of  the  dry  air. 

Dry  Air  at  the  earth's  surface  consists  essentially  of 
nearly  21%  of  oxygen,  about  78%  of  nitrogen,  about  i% 
of  argon,  and  0.03  %  of  carbon  dioxide,  by  volume. 

7 


8  ELEMENTARY   METEOROLOGY. 

Oxygen.  —  There  is  a  slight  decrease  in  the  proportion 
of  oxygen  at  distances  of  several  miles  above  the  sea 
level ;  and  even  near  sea  level  the  amount  of  oxygen  is 
not  absolutely  invariable,  but  the  variations  are  very  slight. 
Oxygen  is  the  most  important  element  of  the  air. 

Nitrogen,  which  forms  the  greater  part  of  the  air,  seems 
to  have  no  special  function  except  to  dilute  the  oxygen. 
It  is  slightly  lighter  than  oxygen. 

Argon  enters  into  such  close  combination  with  pure 
nitrogen,  that  it  has  always,  until  its  recent  discovery, 
been  included  with  the  atmospheric  nitrogen.  Its  peculiar 
function  has  not  yet  been  ascertained.  It  is  the  most 
dense  of  the  gaseous  constituents  of  the  air. 

Carbon  Dioxide,  which  is  a  compound  of  oxygen  and 
carbon,  varies  slightly  in  amount,  being  about  3  parts  in 
10,000  of  air  on  the  average ;  but  there  seems  to  be  a 
little  more  in  cloudy  than  in  clear  weather,  and  a  little 
more  at  night  than  by  day.  Carbon  dioxide,  though  it 
forms  such  a  small  proportion  of  the  air,  is  a  very  important 
factor  in  the  life  on  the  earth,  since  it  forms  the  basis  of 
food. 

All  animal  organisms  inhale  oxygen,  and  exhale  carbon 
dioxide.  Carbon  dioxide  is  frequently  emitted  as  a  gas 
from  volcanoes,  in  such  quantities  as  to  render  the  air  into 
which  it  passes  too  impure  for  breathing  purposes,  espe- 
cially in  low-lying  adjacent  places  into  which  the  gas  sinks 
on  account  of  its  relatively  great  density. 

Carbon  dioxide  is  of  no  direct  use  to  animals  after  it  has 
been  exhaled  in  the  breathing  process.  It  is,  however,  of 
much  importance  to  plant  life ;  for  the  carbon  is  the  chief 
food  of  plants.  The  carbon  dioxide  of  the  air  is  taken  up 
by  certain  cells  of  the  plant,  and  is  there  decomposed  by 
the  sunlight  into  carbon  and  oxygen.  The  oxygen  escapes 


THE   EARTH'S   ATMOSPHERE.  9 

into  the  air,  but  the  carbon  is  retained  to  build  up  the  tis- 
sues of  the  plant. 

Microscopical    Impurities    in    the    Atmospheric    Air, — 

Minute  solid  particles  of  matter  are  found  in  the  air, 
and  these  may  be  divided  into  two  classes,  —  inorganic 
and  organic. 

The  inorganic  particles  are  the  dust  particles,  which  we 
see  floating  in  a  ray  of  sunlight  as  it  crosses  a  darkened 
room ;  and  the  smoke  particles,  which  in  the  aggregate 
are  visible  as  smoke.  The  dust  and  smoke  particles  are 
of  great  importance  in  the  changing  of  the  water  vapor  in 
the  atmosphere  into  drops  of  water. 

The  number  of  dust  particles  in  the  air  varies  enormously  at  different 
times  and  places.  The  number  is  least  and  most  constant  in  clear 
weather,  and  greatest  and  most  variable  in  cloudy  weather ;  it  is  least 
over  the  ocean  and  at  great  distances  above  the  surface  of  the  earth ; 
and  the  size  of  the  particles  in  general  decreases  with  the  height  above 
this  surface.  On  the  top  of  a  high  mountain,  about  a  mile  above  sea 
level,  it  was  found  that  there  were  11,000  dust  particles  per  cubic  inch 
when  the  air  was  clear ;  but  the  number  increased  to  50,000  or  65,000 
during  the  temporary  fog  produced  by  the  passage  of  an  isolated  cloud 
over  the  mountain  top. 

The  organic  particles  in  the  air  are  the  minute  germs 
Called  microbes.  These  are  of  two  classes,  —  the  bacteria 
and  the  molds.  The  bacteria  are  the  minute  forms  of 
animal  life,  such  as  the  disease  germs ;  and  they  are  most 
frequent  in  the  air  of  hospitals,  and  least  frequent  over 
the  oceans  and  on  mountain  tops.  The  molds  are  minute 
forms  of  vegetable  life  (fungi)  which  occasion  fermenta- 
tion and  the  decomposition  of  organic  matter.  They  ar^e 
most  numerous  in  foul  damp  air,  such  as  that  found  in 
sewers.  Organic  particles  vary  in  number  with  the  sea- 
son of  the  year  and  the  hours  in  the  day. 


10  ELEMENTARY  METEOROLOGY. 

Density  and  Volume  of  Gases.  —  Gases  are  elastic,  and 
they  may  be  compressed  into  a  small  space ;  but  if  pres- 
sure is  removed,  they  expand,  and  occupy  larger  space. 
With  a  constant  temperature,  the  volume  varies  inversely 
as  the  pressure :  that  is,  if  the  pressure  is  doubled,  the 
volume  is  reduced  one  half.  This  relation  is  maintained 
for  the  air,  within  the  limits  of  meteorological  study,  but  for 
very  great  and  very  small  pressures  it  does  not  hold  good. 

The  pressure  on  any  point  in  a  quiet  gas  will  be  the  same  in  all 
directions,  —  upwards,  downwards,  and  towards  either  side.  The  air 
of  the  atmosphere  is  acted  on  by  the  gravity  which  draws  it  towards 
the  center  of  the  earth,  and  the  amount  of  this 
downward  force  is  the  weight  of  the  air.  When 
we  consider  the  outside  pressures  on  a  quantity 
_Pr  of  free  air,  such  as  may  be  represented  by  the 
cube  shown  in  Fig.  i,  then  the  pressure  Pt  (from 
without)  will  just  equal  the  pressure  Pr ;  but  the 
upward  pressure  Pb  will  exceed  the  downward 
pressure  Pa  by  the  weight  of  the  air,  that  is,  by 
the  downward  pull  g  exerted  by  gravity  on  the  cubic  mass  of  air. 
Then  the  pressure  Pb  =  Pa  +  g.  Likewise  Pa  =  Pb—  g. 

The  side  pressure  Pt  =  ihe  side  pressure  Pr\  but  these  side  pressures 
at  the  upper  edge  of  the  cube  are  each  equal  to  Pa,  and  they  increase, 
by  the  pressure  corresponding  to  the  weight  of  g,  to  Pb  at  the  lower 
edge  of  the  cube. 

Pressure  and  Weight  of  the  Air.  —  The  weight  of  the  at- 
mosphere, or  its  downward  pressure  on  any  surface,  is  the 
result  of  the  aggregate  weight  of  all  the  air  above.  It  is 
evident,  then,  that  as  we  go  upward  from  the  earth's  sur- 
face towards  the  outer  limit  of  the  atmosphere,  the  weight 
of  the  air,  and  consequently  its  downward  pressure,  will 
decrease ;  and  this  decrease  will  be  in  proportion  to  the 
weight  of  the  air  left  below  in  the  ascent.  If  the  air  were 
incompressible,  then  the  decrease  in  weight  and  density 


THE  EARTH'S  ATMOSPHERE.  II 

would  be  the  same,  for  the  same  distance  left  behind 
in  ascending,  at  any  height  in  the  atmosphere.  The  air, 
however,  is  compressible;  and  the  greater  the  weight  of 
the  air  above,  the  more  the  air  below  is  compressed,  and 
the  denser  it  becomes.  Thus,  if  we  start  from  the  outer 
limit  of  the  atmosphere,  where  there  is  supposedly  no  air 
above,  the  first  layer  of  air  will  be  attracted  by  the  force  of 
gravity,  but  there  will  be  no  additional  weight  of  air  press- 
ing down  from  above ;  but  when  we  descend  to  the  second 
layer  of  air,  then  gravity  exerts  its  power  on  this  second 
layer  just  as  it  did  on  the  first,  and  in  addition  the  weight 
of  the  upper  layer  presses  down  on  the  second  layer,  and 
compresses  it,  making  it  denser.  The  air  of  a  third  layer 
will  be  still  more  compressed,  because  it  will  have  the 
weight  of  the  two  upper  layers  pressing  down  on  it  in 
addition  to  the  attraction  of  gravity.  Thus  the  air  in- 
creases in  density  all  the  way  down  to  the  earth's  surface 
by  the  compression  due  to  the  weight  of  the  air  above. 

The  Elastic  Force  of  a  Gas  in  a  given  quiet  condition, 
or  its  expansive  force,  is  equal  to  the  surrounding  pressure 
exerted  on  it.  The  different  gases  of  the  atmosphere  have 
different  elastic  forces  for  the  same  quantity  of  matter; 
but  for  any  gas  the  elastic  force  is  increased  if  the  quan- 
tity and  density  of  the  gas  are  increased,  provided  the 
temperature  remains  unchanged. 

The  elasticity  of  a  gas  increases  as  the  gas  grows  warmer, 
and  is  measured  by  its  pressure  in  the  given  or  existing  con- 
dition. For  measurements  of  the  atmospheric  pressure  and 
of  the  separate  gases  which  are  in  the  air,  the  pressure  or 
weight  of  a  column  of  mercury  under  given  conditions  is 
used;  the  pressure  being  measured  by  the  length  of  the 
mercury  column,  in  inches,  which  is  necessary  to  counter- 
poise the  expansive  force  of  the  gas.  The  instrument  used 


12  ELEMENTARY  METEOROLOGY. 

for  such  measurements,  and  called  the  barometer,  is  ex- 
plained in  the  proper  connection  farther  along. 

Arrangement  of  the  Gases  forming  the  Air.  —  Each  of 
the  gases  forming  the  air  has  a  different  degree  of  elas- 
ticity, and  the  same  quantity  of  each  occupies  different 
volumes  under  similar  outside  pressures :  so  that,  the 
greater  the  elastic  force  (pressing  outward),  the  less  will 
the  gas  be  compressed  for  a  given  pressure  from  the  out- 
side ;  and  consequently  its  density  will  be  the  less,  since 
a  smaller  quantity  or  mass  will  suffice  to  neutralize  the 
outside  pressure. 

In  the  case  of  our  atmosphere,  the  pressure  at  any 
height  above  sea  level  is  due  to  the  weight  of  the  mass 
of  the  gas  above :  consequently  the  elasticity  or  pressure, 
and  the  resulting  density,  decrease  with  the  height,  because, 
the  higher  up  we  go,  the  less  gas  there  is  to  press  down 
from  above. 

If  the  air  were  replaced  by  a  single  gas,  then  the 
greater  the  elasticity  of  this  gas,  the  higher  it  would 
extend  above  the  surface  of  the  earth. 

Each  of  the  various  gases  forming  the  air  has  the  same 
distribution  and  arrangement  of  its  parts  which  it  would 
have  if  it  alone  composed  the  atmosphere,  so  that  the 
gases  with  the  least  density  extend  upward  the  farthest. 
Of  the  gases  nitrogen,  oxygen,  and  carbon  dioxide,  nitro- 
gen is  the  least  dense,  and  consequently  extends  up  far- 
ther than  oxygen,  and  much  farther  than  carbon  dioxide 
and  argon,  which  are  most  dense. 

It  has  been  calculated,  from  our  knowledge  of  the  density 
of  the  various  gases  at  the  surface  of  the  earth  or  sea  level, 
and  the  rapidity  with  which  the  density  of  each  decreases 
with  ascent  above  this  surface,  that  the  carbon  dioxide 
practically  disappears  from  the  atmosphere  at  a  height 


THE  EARTH'S  ATMOSPHERE.  13 

of  about  10  miles  above  the  sea  level,  the  water  vapor 
practically  disappears  at  a  height  of  12  miles,  the  oxygen 
gas  disappears  at  a  height  of  30  miles,  and  the  nitrogen 
gas  disappears  at  a  height  of  35  miles. 

Observations  indicate,  however,  that  the  atmosphere  ex- 
tends much  higher  than  this,  which  shows  that  our  knowl- 
edge of  the  laws  of  atmospheric  gases  is  still  incomplete. 

Extreme  Limit  of  the  Air.  —  It  is  probable  that  there  is 
no  definite  outer  limit  to  the  atmosphere,  where  we  can 
say  that  the  air  ceases  and  there  exists  only  the  ether 
which  is  supposed  to  occupy  all  space.  We  have  seen 
that  theoretically  there  should  be  no  air  at  an  elevation  of 
40  miles ;  but  meteors  have  been  observed  at  a  height  of 
loo  miles  or  more,  and  these  are  supposed  to  be  rendered 
luminous  by  the  friction  of  their  passage  through  the  air. 

Peculiar  luminous  clouds  have  also  occasionally  been  observed  at 
altitudes  of  upwards  of  40  or  50  miles ;  and  if,  as  is  supposed,  they  are 
due  to  crystallization  of  water  vapor,  then  moisture  also  exists  at  much 
higher  elevations  than  the  present  theories  will  allow. 

Weight  of  the  Air.  — The  actual  weight  of  a  quantity  of 
air  is  variable  according  to  its  density.  The  greater  the 
density,  that  is,  the  more  the  air  is  compressed,  the  greater 
is  its  weight  for  any  specified  volume.  At  sea  level,  water 
weighs  about  840  times  as  much  as  the  same  bulk  of  air, 
a  cubic  foot  of  air  weighing  about  i|  ounces.  With  in- 
crease of  height  above  the  sea  level,  a  cubic  foot  of  air 
weighs  less  and  less. 

The  Atmospheric  Air  Mass  Relative  to  the  Earth.  — The 
diameter  of  the  earth  is  about  7,900,  and  the  area  of  its 
surface  197,000,000  square  miles.  Since  the  atmosphere 
does  not,  according  to  our  most  delicate  tests,  extend  above 
IOO  miles  from  the  earth's  surface,  then  its  entire  thick- 


14  ELEMENTARY    METEOROLOGY. 

ness  is  only  about  -fa  of  the  radius  of  the  earth ;  but  the 
lower  part  of  the  atmosphere  is  so  much  denser  than  the 
upper  part,  that  one  half  the  total  weight  of  the  atmos- 
phere is  limited  to  the  first  three  miles  above  the  sea  level, 
while  the  other  half  comprises  the  remainder  of  the  atmos- 
phere to  its  outer  limit.  The  thickness  of  the  lower  layer, 
containing  half  the  total  amount  of  air,  is,  then,  less  than 
ToVo  °f  tne  radius  of  the  earth.  If  the  earth  were  repre- 
sented by  a  globe  100  feet  in  diameter,  this  lower  half  of 
the  air  covering  its  surface  would  have  a  thickness  of  less 
than  J  of  an  inch.  The  atmosphere,  then,  must  be  re- 
garded as  a  thin  gaseous  layer  spread  out  over  a  very 
large  spheroid. 

Distribution  of  the  Air  over  the  Earth.  —  The  atmos- 
phere has  not  only  length  and  breadth,  covering  the  whole 
earth's  surface,  but  it  has  also  thickness,  extending  from 
the  earth's  surface  up  to  some  unknown  height.  The 
geographical  terms  latitude  and  longitude  can  be  used  for 
expressing  the  horizontal  extent  of  the  atmosphere,  or  for 
locating  a  mass  of  air  at  any  point  on  the  earth's  surface ; 
but  when  some  point  above  the  earth's  surface  is  to  be  con- 
sidered, then  it  is  necessary  also  to  express  distances  in 
the  direction  of  the  thickness  of  the  atmosphere,  which  is 
done  by  altitudes. 

Altitude.  —  By  the  altitude  of  a  place,  or  of  a  point  in 
the  atmosphere,  is  meant  its  height  above  the  level  of 
the  ocean.  Altitudes  are  expressed  in  feet,  yards,  or 
miles  in  the  English  system  of  measurements,  and  by 
meters  or  kilometers  in  the  metric  system.  Altitudes  are 
also  sometimes  expressed  in  direction  merely,  without  ref- 
erence to  the  absolute  elevation  of  the  point  or  object  to 
be  located.  This  is  accomplished  by  means  of  angular 
measure,  in  which  the  altkude  is  the  angle  between  a  line 


THE   EARTH'S   ATMOSPHERE.  15 

drawn  from  the  observer  to  the  object,  and  another  line 
drawn  from  the  observer  to  the  unobstructed  horizon.  The 
angular  altitude  of  the  horizon  is  o° ;  and  of  the  zenith,  or 
point  directly  overhead,  90°. 

Importance  of  Altitude  in  Meteorology.  —  With  increase 
or  decrease  of  height  above  sea  level,  that  is,  with  vari- 
ation of  altitude,  the  atmospheric  air  undergoes  rapid  and 
marked  changes  in  its  condition.  At  places  on  the  ocean  the 
altitudes  at  which  man  lives  are  approximately  the  same ; 
but  on  the  land  this  is  not  the  case,  for  there  we  find  him 
inhabiting  regions  at  all  altitudes  from  the  low  lands  up  to 
mountain  tops  reaching  15,000  feet  above  the  sea  level. 
Altitude  thus  becomes  a  very  important  matter  in  meteor- 
ology. 

Latitude.  —  The  heat  from  the  sun  is  the  chief  cause  of 
variations  in  atmospheric  conditions :  and  since  the  aver- 
age position  of  the  sun  is  the  plane  of  the  equator,  where 
the  average  angular  altitude  of  the' sun  is  greatest,  and  the 
point  on  the  earth  where  the  suri  has  the  least  average 
angular  altitude  is  at  the  poles,  the  effect  of  the  sun  is 
felt  most  at  the  equator,  and  least  at  the  poles ;  and  the 
various  gradations  between  these  extremes  are  measured 
by  any  scale  marking  the  distance  between  the  equator 
and  the  poles.  Such  a  measure  we  have  in  the  degrees 
of  latitude,  which  divide  the  distance  into  90  equal  parts. 
But  since  the  degrees  of  latitude  are  counted  as  increas- 
ing from  the  equator  towards  the  pole,  —  the  latitude  of 
the  equator  being  o°,  and  that  of  the  pole  being  90°,  — 
then,  as  the  latitude  increases,  the  influence  of  the  sun  on 
the  atmosphere  decreases. 

Longitude.  —  In  the  direction  transverse  to  latitude,  that 
is,  in  the  direction  in  which  we  use  longitude  in  geograph- 
ical measurements,  there  is  no  such  absolute  permanent 


1 6  ELEMENTARY   METEOROLOGY. 

variation  in  the  atmospheric  conditions  brought  about  by  the 
solar  heat ;  and  so  the  degrees  of  longitude  cannot  be  used 
in  expressing  a  law  of  physical  change  such  as  has  just 
been  indicated  for  the  degrees  of  latitude.  But  still,  longi- 
tude has  its  usual  geographical  significance  in  locating 
points  on  the  earth's  surface,  the  meteorological  conditions 
of  which  it  may  be  desired  to  consider. 

Longitude,  however,  is  used  in  another  way,  and  that  is, 
as  representing  time.  We  know  that  the  rotation  of  the 
earth  on  its  axis  causes  the  sun  to  appear  to  travel  around 
the  earth  in  twenty-four  hours.  Now,  this  distance  around 
the  earth  is  360°  of  longitude,  so  that  the  apparent  motion 
of  the  sun  is  through  1 5°  of  longitude  in  one  hour.  The  sun, 
then,  in  pursuing  its  apparent  course  in  the  heavens,  gets 
back  to  the  same  position  every  twenty-four  hours.  For 
any  one  place  there  is  thus  a  return  to  practically  the  same 
conditions,  so  far  as  the  sun  is  concerned,  at  the  expiration 
of  each  twenty-four  hours,  no  matter  what  changes  may 
have  been  experienced  in  the  intervening  hours.  Such  a 
circuit  is  called  a  cycle. 

Meteorological  Elements.  —  The  different  items  by  which 
the  total  meteorological  condition  of  the  atmosphere  (at 
any  place)  is  represented,  are  called  the  meteorological 
elements.  These  are,  — 

The  temperature  of  the  air,  or  its  degree  of  heat. 

The  pressure  of  the  air,,  or  its  amount  or  quantity,  and 
density. 

The  humidity,  or  amount  of  water  contained  in  the  air. 

The  precipitation,  or  amount  of  water  which  the  air  loses 
as  rain  or  snow. 

The  evaporation,  or  amount  of  water  which  the  air  takes 
up  from  the  earth. 

The  windy  or  the  movement  of  the  air. 


THE   EARTH'S   ATMOSPHERE.  I/ 

The  clouds,  or  the  obscuration  of  the  sky. 

The  electrical  and  optical  conditions  of  the  air. 

Meteorological  Conditions.  —  Observations  of  the  meteor- 
ological elements  show  that  they  do  not  long  remain  in 
the  same  condition,  but  are  continually  undergoing  changes. 
These  changes  are  of  two  classes,  —  regular  or  periodic, 
and  irregular  or  accidental. 

A  periodic  change  is  one  in  which  the  element  returns  to 
substantially  the  same  condition  after  the  lapse  of  periods 
of  time  of  approximately  uniform  length.  The  principal 
periods  are  the  diurnal  or  daily,  depending  on  the  rotation 
of  the  earth  upon  its  axis ;  and  the  annual  or  yearly,  de- 
pending on  the  revolution  of  the  earth  about  the  sun. 

An  accidental  change  is  one  which  occurs  at  irregular 
intervals,  and  whose  time  of  occurrence  cannot  be  foretold. 

The  average  condition  of  any  meteorological  element  dur- 
ing a  given  time  is  obtained  by  finding  the  sum  of  a  number 
of  single  observations  of  its  condition  made  during  this 
time  (usually  at  equal  short  intervals  of  time),  and  dividing 
this  sum  by  the  number  of  observations. 

In  addition  to  the  average  condition,  it  is  also  of  interest 
to  know  the  extreme  fluctuations  which  may  occur  in  the 
meteorological  elements  during  both  periodic  and  acci- 
dental changes.  The  maximum  amount  is  that  which  the 
phenomenon  attains  when  it  reaches  its  greatest  value. 
The  minimum  amount  is  that  which  it  attains  when  it 
reaches  its  least  value.  The  amplitude  is  the  difference 
between  the  maximum  and  the  minimum  amounts. 

Special  conditions  are  called  phases.  The  various  phases 
of  meteorological  conditions  are  investigated  as  regards 
the  time  of  occurrence  as  well  as  the  amount  or  quantity. 

Meteorological  Instruments.  —  In  order  to  observe  with 
the  required  accuracy  the  meteorological  conditions,  it  is 

WALDO    METEOR.  —  2 


1 8  ELEMENTARY   METEOROLOGY. 

necessary  to  have  instruments  properly  adapted  for  mak- 
ing measurements  of  the  different  meteorological  elements. 
These  instruments  are  of  two  classes.  The  first  class  is 
used  by  an  observer  for  the  direct  observation  of 'condi- 
tions. The  second  class  is  used  for  obtaining  an  automatic 
registration  of  conditions  by  mechanical  or  photographic 
means :  these  instruments  are  usually  of  very  complicated 
construction. 

The  Distribution  of  Meteorological  Elements  is  the^  condi- 
tion of  the  meteorological  elements  at  different  localities 
during  the  same  phases  or  periods  of  time.  It  is  usually 
presented  by  means  of  geographical  charts  on  which  are 
entered  the  conditions,  each  at  its  proper  locality  on  the 
map. 

Statical  Meteorology  treats  of  the  conditions  of  the  me- 
teorological elements  without  considering  the  changes  in 
the  air  due  to  its  motion  oi  translation.  It  therefore  em- 
braces the  whole  matter  of  the  constitution  of  the  atmos- 
phere, and  such  conditions  as  have  the  question  of  space, 
but  not  of  time,  enter/ing  into  them ;  as,  f  or^  instance,  -the 
average  values  of  the  meteorological  elements  for  any 
period,  or  their  condition  at  any  chosen  instant.  c 

Dynamical  Meteorology  treats  of  /  the  motions  of  the 
atmospheric  air,  their  causes,  and  the  conditions  arising 
therefrom,  and  also  of  the  modification  which  these 
motions  cause  in  the. statical  conditions. 


CHAPTER   II. 
TEMPERATURE. 

Heat  is  due  to  very  rapid  motions  of  the  minute  particles, 
called  molecules,  of  which  bodies  are  composed.  Heat  is 
not  a  substance,  but  is  a  form  of  energy.  It  may  be  trans- 
ferred from  one  body  to  another.  Heat  of  various  degrees 
or  intensities  may  exist,  and  these  intensities  are  measurable. 

Temperature  is  a  general  term  applied  to  express  the 
intensity  or  degree  of  heat.  We  thus  say  the  temperature 
of  the  body  is  high  or  low,  according  as  the  body  is  hot  or 
cold.  The  instrument  used  for  making  accurate  measure- 
ments of  the  temperature  of  bodies  is  called  a  thermometer, 
or  heat  measurer. 

Diffusion  of  Heat.  —  The  process  by  which  heat  is  trans- 
ferred from  one  body  to  another,  or  from  one  part  to 
another  part  of  the  same  body,  is  called  the  diffusion  of 
heat.  The  diffusion  of  heat  always  takes  place  by  the 
transference  of  heat  from  a  hotter  to  a  colder  body,  or 
from  a  hotter  to  a  colder  part  of  the  same  body.  The  dif- 
fusion of  heat  is  accomplished  by  conduction,  convection, 
radiation,  or  reflection,  or  by  some  combination  of  these. 

Conduction  is  the  flow  of  heat  from  the  hotter  to  the 
colder  places  in  an  unequally  heated  body  or  in  adjacent 
bodies.  The  thermal  conductivity  of  a  .body  is  its  capacity 
for  the  conduction  of  heat  through  it. 

The  conduction  of  heat  does  not  take  place  with  the  same  facility  or 
rapidity  in  all  bodies.  The  metals  are  good  conductors,  silver  espe- 

19 


2O  ELEMENTARY  METEOROLOGY. 

dally,  as  may  be  realized  by  putting  one  end  of  a  silver  spoon  into  hot 
water,  and  holding  the  other  end  in  the  hand.  Air,  stone,  water,  ice, 
snow,  wood,  and  wool  are  poor  conductors.  The  reason  why  woolen 
clothing  is  so  desirable  in  winter  is  that  it  is  a  poor  conductor,  and  so 
the  heat  of  the  body  is  retained. 

Convection  is  the  transference  of  the  heat  by  the  circula- 
tion of  the  hot  body  itself,  by  which  it  is  brought  succes- 
sively in  contact  with  colder  bodies.  In  this  process  the 
final  transference  of  heat  from  the  one  body  to  the  other 
usually  takes  place  by  conduction.  Thermal  convection 
takes  place  in  fluids. 

Radiation  is  the  transference  of  heat  from  a  hot  body 
through  a  medium  which  does  not  itself  become  much 
heated  in  the  process,  but  which  must  be  colder  than 
the  hot  body.  The  diathermancy  of  a  body  is  its  capacity 
for  the  transmission  of  radiant  heat  without  itself  becom- 
ing heated. 

In  the  passage  of  heat  through  a  body,  some  of  the  heat 
is  always  retained  by  the  body,  and  this  retention  is  called 
thermal  absorption. 

Reflected  Heat  is  the  heat  immediately  thrown  off  from 
the  surface  of  a  body  without  entering  it,  when  it  is  receiv- 
ing heat  by  radiation  from  another  body. 

Radiant  Energy.  —  Energy  is  radiated  from  the  sun,  and 
is  transmitted  to  our  earth  by  a  vibratory  process,  through 
the  ether  which  fills  space.  The  slower  vibrations  are 
rendered  sensible  to  us  as  heat,  and  the  more  rapid  ones 
as  light. 

Heat  of  the  Atmosphere.  —  In  meteorology  we  have  to  deal 
principally  with  the  temperatures  at  and  near  the  earth's 
surface.  There  heat  is  received  from  two  principal  sources, 
viz.,  the  sun,  and  the  interior  of  the  earth  itself.  The  heat 
from  the  sun  we  call  the  solar  heat,  and  that  from  the 


TEMPERATURE.  2 1 

earth  terrestrial  heat.  The  combined  action  of  the  solar 
and  terrestrial  heat  is  what  we  measure  when  we  obtain 
the  temperature  of  the  air  with  which  we  are  usually 
brought  into  actual  contact.  If  we  descend  considerably 
below  the  surface  of  the  earth,  as  into  a  mine,  the  influence 
of  the  heat  of  the  earth  itself  is  more  strongly  felt ;  while 
at  and  above  the  earth's  surface  the  heat  from  the  sun  is 
more  noticeable.  The  solar  heat  is  of  most  importance  in 
meteorology,  and  it  is  the  solar  heat  which  maintains  the 
animal  and  vegetable  life  on  the  earth. 

Solar  Heat  and  Solar  Radiation.  —  The  sun  is  a  very 
large  body,  about  880,000  miles  in  diameter,  and  it  is  in- 
tensely hot.  The  space  around  the  sun  is  very  cold,  and, 
according  to  the  law  of  the  transmission  or  diffusion  of  heat, 
the  sun  must  lose  a  portion  of  its  heat  in  the  endeavor 
to  equalize  its  own  heat  and  that  of  the  surrounding 
space.  Heat  is  transmitted  radially  (that  is,  straight  out, 
like  the  spokes  from  the  hub  of  a  wheel)  from  the  sun 
through  this  space  by  radiation ;  and,  unless  some  matter 
interposes,  these  solar  rays  as  they  are  called  will  keep 
on  indefinitely.  Our  earth,  in  its  revolution  around  the 
sun,  intercepts  less  than  one  half  of  a  billionth  of  the 
whole  amount  of  heat  given  off  by  the  sun,  yet  the  amount 
received  is  amply  sufficient  for  the  purpose  of  sustaining 
animal  and  vegetable  life  on  the  earth.  Although  the 
sun  is  steadily  parting  with  such  vast  quantities  of  heat, 
the  most  careful  observations  fail  to  show  that  it  is  be- 
coming appreciably  cooler. 

Revolution  of  the  Earth  around  the  Sun.  —  The  earth 
moves  around  the  sun  in  one  year  in  a  slightly  elliptical 
path  or  orbit,  the  sun  being  not  in  the  center,  but  in  one 
of  the  foci  of  the  orbit.  The  earth  is  therefore  at  times 
slightly  nearer  to  the  sun  than  at  other  times.  The 


22  ELEMENTARY   METEOROLOGY. 

average  distance  of  the  earth  from  the  sun  is  about 
92,890,000  miles.  On  Jan.  i,  when  the  earth  is  nearest 
the  sun,  it  is  91,300,000  miles  from  it;  and  on  July  i, 
when  it  is  farthest  away,  it  is  94,450)000  miles  from  it.  If 
the  earth  moved  around  the  sun  in  a  circular  path,  it 
would  receive  a  constant  total  amount  of  heat  and  light 
during  every  day  of  the  year;  but,  as  it  is,  the  amount 
received  on  Jan.  i  is  7  %  greater  than  that  received  on 
July  i. 

It  requires  365.2422  days,  or  nearly  365^  days,  for  the 
earth  to  make  a  complete  revolution  around  the  sun.  Its 
path  lies  in  a  plane  called  the  plane  of  the  ecliptic, 

Inclination  of  the  Earth's  Axis  of  Rotation.  —  The  earth 
has  a  rotation  once  in  24  hours  around  an  axis  passing 
through  its  center.  This  axis  passes  through  the  north 
and  south  poles  of  the  earth,  and  is  inclined  at  an  angle 
of  about  66|°  to  the  plane  of  the  ecliptic,  or  plane  of 
revolution  of  the  earth  around  the  sun ;  so  that,  when 
the  earth  revolves  around  the  sun,  this  axis  does  not  stand 
perpendicular  to  this  plane  of  revolution,  but  is  inclined 
23^°  to  this  perpendicular.  This  is  a  most  important  mat- 
ter in  the  formation  of  meteorological  conditions,  because 
it  controls  or  marks  out,  to  a  great  extent,  the  manner  and 
places  in  which  the  solar  rays  reach  the  earth's  surface. 

Effects  of  the  Inclination  of  the  Earth's  Axis  on  the  Dis- 
tribution of  Solar  Rays  on  the  Earth's  Surface.  —  If  the 
earth's  axis  were  perpendicular  to  the  plane  of  the  ecliptic, 
the  length  of  the  days  and  nights  would  be  equal,  and  the 
plane  of  the  equator  would  coincide  with  that  of  the  eclip- 
tic ;  so  that  during  the  entire  year  the  apparent  path  of 
the  sun  would  lie  along  the  equator.  But  with  the  con- 
stant inclination  of  the  axis  the  following  conditions  result 
during  a  revolution  of  the  earth  around  the  sun  :  — 


TEMPERATURE. 


23 


FIG.  2.  —  PORTIONS  OF  THE  EARTH  RECEIVING 

THE  SoLAR  RAYS> 


The  sun  shines  over  a  whole  hemisphere  at  all  times  ; 
but  the  part  of  the  earth  receiving  the  solar  rays  varies 
with  the  position  of  the  earth  in  its  orbit.  The  earth  has 
the  position  B  (Fig.  2)  on  Dec.  21,  C  on  March  21,  D 
on  June  21,  and  A  on  Sept.  22.  We  can  consider  that 
when  the  sun  is  over  the 
earth's  equator,  and  con- 
sequently the  earth's  po- 
lar axis  is  turned  90° 
from  the  sun,  it  is  in  its 
normal  or  average  posi- 
tion.  On  Dec.  21  the 
earth  stands  so  that  the 

,  ,  i     ,. 

north  pole  is  turned  far- 
ther  away  from  the  sun 

by  the  full  inclination  of  the  pole  to  the  perpendicular  to 
the  ecliptic,  or  at  an  angle  of  23^°,  and  the  sun  shines  on 
the  south  pole  and  23^°  beyond  it  (B).  On  June  21  the 
opposite  extreme  is  reached  ;  and  the  north  pole  is  turned 
towards  the  sun,  and  the  south  pole  away  from  it,  by  this 
same  angle,  and  the  sun  shines  on  the  north  pole  and  23^° 
beyond  it  (D).  Halfway  between  these  dates  the  condi- 
tions become  as  shown  at  A  and  C,  where  the  region 
receiving  the  solar  rays  reaches  from  pole  to  pole. 

In  the  daily  rotation  of  the  earth  when  in  the  positions 
A  and  C,  the  sun  appears  to  move  through  the  sky  in  the 
plane  of  the  equator.  In  the  position  D  it  appears  to 
move  in  the  plane  of  the  Tropic  of  Cancer  (23^°  north 
latitude).  In  the  position  B  it  appears  to  move  in  the 
plane  of  the  Tropic  of  Capricorn  (23|-°  south  latitude). 
The  change  from  the  position  A  to  B,  thence  to  C,  and 
thence  to  D,  and  back  again  to  A,  takes  place  gradually, 
little  by  little  each  day,  as  the  earth  moves  around  the  sun. 


ELEMENTARY   METEOROLOGY. 


Periods  of  Presence  and  Absence  of  Solar  Radiation  at 
Places  on  the  Earth's  Surface.  —  Only  half  of  the  earth 
can  receive  the  solar  rays  at  any  time,  and,  as  we  have 
just  seen,  the  region  receiving  these  rays  varies  constantly 
as  the  earth  moves  forward  in  its  orbit.  When  the  solar 
rays  reach  from  pole  to  pole,  at  the  points  A  and  £T(Fig.  2) 
the  days  and  nights  are  of  equal  length  (12  hours  long) 
all  over  the  world.  During  the  time  when  the  earth  is 
passing  from  A  to  C  through  B,  the  solar  rays  reach 
beyond  the  south  pole,  but  fail  to  reach  the  north  pole  by 
a  like  amount,  and  the  daytime  is  lengthened,  and  the 
nighttime  shortened,  in  the  southern  hemisphere ;  and 
for  a  varying  region,  reaching  a  maximum  of  23^°  at  B, 
there  is  uninterrupted  day  around  the  south  pole,  and 
uninterrupted  night  around  the  north  pole. 

Similarly,  when  the  earth  is  passing  from  C  to  A 
through  D,  the  daytime  is  longer  and  the  nighttime 
shorter  in  the  northern  hemisphere ;  and  in  a  variable 
region,  reaching  a  maximum  at  D,  there  is  continual  day 
around  the  north  pole,  and  continual  night  around  the 
south  pole. 

The  days  are  longer  than  the  nights  in  the  polar  hemi- 
sphere which  is  receiving  the  most  solar  rays ;  but  at  the 
equator  the  days  and  nights  are  always  the  same. 

The  following  table  shows  the  greatest  possible  length  of  the  con- 
tinued or  uninterrupted  visibility  of  the  sun  at  various  latitudes  on  the 
earth  :  — 


Latitude  
Hours  of  visibility 

0° 

12* 

I7° 

i3h 

41° 

'5h 

49° 

i6h 

63° 

20h 

66°5' 
24" 

67°2i' 
i  mo. 

69°5i 
2  mo. 

78°!  i' 
4  mo. 

90° 

6  mo. 

The  twilight,  which  is  the  transition  period  between  daylight  and 
darkness,  also  increases  in  length  with  the  distance  from  the  equator. 
Thus,  of  the  8,766  hours  which  make  up  a  year,  there  are, — 


TEMPERATURE.  25 

AT  THE  EQUATOR.  AT  THE  POLES. 

4,407  hours  day.  4»45°  hours  day. 

864  hours  twilight.  2,403  hours  twilight. 

3,495  hours  night.  1,913  hours  night. 

The  Apparent  Motion  of  the  Sun  around  the  Earth.  —  The  various 
phenomena  just  described  as  due  to  the  rotation  of  the  earth  on  its 
axis,  the  inclination  of  the  axis  to  the  ecliptic,  and  the  revolution  of  the 
earth  around  the  sun,  are  best;  realized  by  considering  the  matter  as  it 
appears  to  us. 

The  sun  appears  to  revolve  around  the  earth  once  in  24  hours, 
and  on  March  21  this  revolution  takes  place  in  the  plane  of  the 
equator ;  but  the  sun  has  a  slow  apparent  movement  towards  the  north 
at  the  same  time  that  it  moves  around  the  earth  ;  and  the  result  of 
these  two  motions  is  a  spiral  movement,  which  carries  the  sun  across 
the  successive  parallels,  so  that  by  June  21  it  revolves  around  the  earth 
on  the  parallel  of  23  £°  north  latitude.  During  this  time  the  sun  has 
apparently  revolved  around  the  earth  91  times.  After  this  date  there 
is  a  spiral  motion  southward,  which  gradually  carries  the  sun  back  to 
the  plane  of  the  equator  on  Sept.  22.  The  sun  then  crosses  the  equator 
into  the  southern  hemisphere,  and  pursues  its  spiral  course  until  it 
reaches  the  plane  of  237°  south  latitude  on  Dec.  21,  when  it  recedes 
again  towards  the  equator,  reaching  it  on  March  21.  As  the  sun  thus 
revolves  around  the  earth  (very  nearly  in  the  planes  of  successive  par- 
allels), it  always  rises  in  the  east,  reaching  the  zenith  at  noon,  and  sets 
in  the  west,  of  places  along  the  parallel  of  latitude  on  which  it  happens 
to  be.  Thus,  on  March  21  and  Sept.  22,  the  sun  is  directly  in  the 
zenith  of  the  equator  at  noon  ;  at  noon  on  Dec.  21  it  is  directly  in  the 
zenith  at  latitude  23^°  south,  and  on  June  21  at  latitude  23^°  north. 

Amount  of  Solar  Heat  which  would  reach  the  Earth's 
Surface  in  the  Absence  of  the  Atmosphere.  —  The  sun's 
heat  which  would  reach  the  earth's  surface,  were  there  no 
atmosphere,  would  vary  in  intensity  at  different  places 
with  the  angle  at  which  the  rays  struck  the  earth's  sur- 
face. When  the  sun's  rays  lay  parallel  with  the  surface, 
which  would  be  the  case  for  any  special  locality  on  the 
atmosphereless  earth  when  the  sun  was  at  the  horizon,  the 


26  ELEMENTARY   METEOROLOGY. 

amount  of  solar  heat  received  at  that  locality  would  be 
zero.  When  the  sun's  rays  struck  the  surface  squarely, 
which  would  be  the  case  when  the  sun  reached  the  zenith, 
the  intensity  would  reach  its  greatest  value. 
In  Fig.  3,  the  rays  received  between  c  and 
d  are  greatest,  and  those  between  a  and 
b  or  e  and  /are  least,  in  intensity. 

At  the  time  of   the  equinoxes,  when 
FIG.  3.—  RELATIVE  IN-    the    sun    is    directly    over    the    equator, 

TENSITY  OP  SOLAR  RAVS. 


surface  at  the  equator  with  their  maximum  strength  ;  but 
at  the  poles  they  would  be  parallel  to  the  surface,  and 
therefore  their  effect  there  would  be  zero  :  and  the  heat- 
ing power  of  the  sun's  rays  would  decrease,  then,  from 
the  equator  to  the  poles,  or,  as  it  is  usually  expressed, 
would  decrease  inversely  as  the  latitude. 

When  the  sun  is  over  the  equator,  if  the  amount  of  solar  heat  at  the 
equator  were  put  at  i.oo,  then  at  the  pole  it  would  be  o.oo.  But  since 
the  sun  departs  from  the  equator  towards  the  south  23^°  at  one  season 
of  the  year,  and  a  like  amount  towards  the  north  at  another  season  of 
the  year,  and  since  the  distance  of  the  earth  from  the  sun  does  not  re- 
main a  constant  quantity,  it  becomes  a  very  complex  matter  to  calculate 
the  relative  amounts  of  heat  actually  received  at  points  on  the  earth's 
surface  during  any  24  hours. 

With  the  increase  of  latitude  there  is  a  lengthening  of  the  number 
of  hours  a  day  during  which  the  sun  shines  in  summer,  and  this  makes 
the  total  amount  of  heat  received  in  higher  latitudes  during  the  daytime 
much  greater  than  one  would  suppose.  Thus,  when  the  sun  reaches  its 
greatest  altitude  at  the  poles,  and  shines  there  continually,  the  amount 
of  heat  received  there  during  24  hours  is  over  20%  more  than  that 
received  during  24  hours  at  the  equator,  when  the  sun  is  above  the 
horizon  for  but  half  the  time. 

If  the  solar  radiation  during  the  average  day  at  the  equator,  which  we 
will  call  the  thermal  day,  were  taken  as  a  unit,  then  365.24  would  repre- 
sent the  annual  solar  radiation  at  the  equator  ;  and  the  annual  solar  radia- 


c     ° 

TEMPERATURE.  2J 

tion  at  other  latitudes  expressed  in  these  same  thermal  days  would  be 
as  follows :  — 


Latitude   .    .    . 
Thermal  days    . 

0° 

365-2 

10° 

360.2 

20° 

34S-2 

30° 
321.0 

40° 
288.5 

50° 
249.7 

60° 
207.8 

70° 
173.0 

80° 
156.6 

90° 
151.6 

It  must  be  distinctly  remembered  that  these  are  only  relative  values. 

The  absolute  quantity  of  solar  heat  which  the  earth  would  receive  dur- 
ing the  year  has  been  calculated  to  be  one  and  one  half  quadrillion  heat 
units,1  or  enough  to  melt  an  ice  layer  over  141  feet  thick,  and  to  evaporate 
a  layer  of  water  nearly  18  feet  deep,  covering  the  whole  earth's  surface. 

Solar  Constant.  —  It  has  been  found  by  'experiments  in  recent 
years,  that,  unaffected  by  the  atmospheric  absorption  and  radiation, 
the  intensity  of  the  solar  rays  falling  vertically  is  such  as  to  heat 
3  grams  of  water  i°  C.  (1.8°  F.)  per  minute  over  each  square  centi- 
meter of  exposed  surface.  This  number  3  is  called  the  solar  constant. 

Solar  Heat  reaching  the  Earth's  Surface  in  the  Presence 
of  the  Atmosphere.  —  When  the  influence  of  the  atmos- 
phere on  the  solar  heat  is  considered,  the  computation  of 
the  amount  of  heat  reaching  the  earth's  surface  becomes 
very  complicated  on  account  of  the  absorption  of  part  of 
the  heat  by  the  air.  The  amount  of  absorption  depends 
principally  on  the  length  of  the  path  of  the  rays  through 
the  air,  the  amount  of  moisture  present,  and  the  degree 
of  cloudiness. 

For  a  perfectly  clear  or  cloudless  sky  when  the  sun  is 
in  the  zenith  of  any  point,  about  75%  of  the  solar  radiation 
will  reach  the  earth's  surface  at  that  point,  and  25%  will  be 
absorbed  by  the  atmosphere.  But  when  the  sun  is  not  in 
the  zenith,  that  is,  when  the  altitude  of  the  sun  is  less  than 
90°,  then  the  thickness  of  the  air  layer  through  which  the 
solar  rays  must  pass  will  be  increased,  and  consequently 
the  loss  of  heat  will  be  greater  ;  while,  when  the  sun  is  just 


1  The  unit  of  heat  is  the  amount  necessary  to  raise  the  temperature  of  one 
cubic  centimeter  of  water  one  degree  Centigrade. 


28 


ELEMENTARY   METEOROLOGY. 


above  the  horizon,  nearly  all  of  the  heat  will  be  absorbed 
by  the  atmosphere,  and  almost  none  will  reach  the  earth's 
surface  at  the  point  of  observation. 

The  following  table  shows  the  proportional  thickness  of  the  air 
layers  through  which  the  solar  rays  pass  at  different  angular  altitudes 
of  the  sun,  taking  the  vertical  thickness  of  the  atmosphere  as  a  unit  or 
i.oo;  and  it  also  shows  in  per  cent  the  amount  of  the  solar  radiation 
which  reaches  the  earth's  surface,  taking  the  amount  received  from  the 
zenith  on  the  outside  of  our  atmosphere  as  i.oo. 


Altitude  of  the  sun  .     .     .    '. 

0° 

c;0 

10° 

20° 

q0o 

C00 

70° 

00° 

Thickness  of  the  atmosphere  in 
units  

qt  t 

IO  2 

c  e6 

2  no 

I  QQ 

I  31 

I  06 

Amount  of  solar  radiation  reach- 
ing the  earth 

O  OO 

O  OCC 

O  2O 

The  solar  rays  possess  different  qualities,  and  are  ab- 
sorbed in  different  proportions  in  their  passage  through 
the  air,  according  to  their  color,  or  position  in  the  spec- 
trum. The  ultra-violet  and  violet  rays  are  absorbed  the 
most ;  next  come  the  indigo,  blue,  green,  yellow,  orange, 
red,  and  ultra-red  in  the  order  named.  The  sun,  therefore, 
has  a  more  bluish  appearance,  the  less  the  thickness  of 
the  atmosphere  through  which  the  solar  rays  pass. 

The  heat  rays  are  much  more  readily  absorbed  than  the 
light  rays ;  so  that,  when  the  sun  is  near  the  horizon,  we 
may  receive  some  of  the  light  rays,  but  almost  none  of  the 
heat  rays.  The  intensity  of  the  sunlight  is  therefore  not  a 
direct  measure  of  the  total  amount  of  solar  radiation. 

In  the  middle  latitudes  in  which  we  live,  in  wholly  clear 
weather,  about  half  of  the  daily  solar  radiation  is  absorbed 
by  the  air;  that  is,  we  receive  only  half  the  amount  of 
heat  that  we  should  receive  if  there  were  no  air,  or  if  the 
air  had  no  effect  on  the  solar  heat. 


TEMPERATURE.  29 

The  solar  rays  which  are  stopped  by  the  air  are  not 
wholly  lost  to  us,  but  we  receive  a  portion  of  them  by  radi- 
ation from  the  air  itself,  and  from  the  particles  of  matter  in 
solid  and  fluid  form  which  the  air  contains.  But  the  great- 
est thermal  use  of  the  atmosphere  is  to  prevent  the  rapid 
radiation  of  heat  from  the  earth's  surface  off  into  space. 

It  has  been  calculated,  that,  if  there  were  no  atmosphere  to  check 
the  radiation  of  heat  into  space,  the  temperature  of  the  earth's  surface 
would  be  about  325°  F.  below  zero. 

Solar  Radiation  on  the  Northern  and  Southern  Hemi- 
spheres. —  In  the  northern  hemisphere,  during  the  period 
when  the  sun  is  north  of  the  equator,  the  intensity  of  the 
solar  radiation  is  slightly  less  than  that  in  the  southern  hemi- 
sphere when  the  sun  is  south  of  the  equator,  because  in  the 
former  case  the  earth  is  at  a  greater  distance  from  the  sun, 
and  the  intensity  of  the  solar  rays  decreases  rapidly  as  this 
distance  increases.  The  summers  on  the  southern  hemi- 
sphere are  consequently  slightly  warmer,  and  the  winters 
slightly  colder,  than  those  of  the  northern  hemisphere,  so  far 
as  the  intensity  of  the  solar  heat  enters  into  the  matter.  In 
other  words,  the  solar  climate,  as  the  climatic  influence  of 
the  sun  is  called,  undergoes  slightly  more  extreme  changes 
in  the  southern  than  in  the  northern  hemisphere.  For  the 
whole  year,  however,  the  northern  and  southern  hemi- 
spheres receive  a  like  amount  of  solar  heat. 

Distribution  and  Transference  of  Solar  Heat  on  the 
Earth's  Surface.  —  The  solar  rays  which  reach  the  sur- 
face of  the  earth  are  partly  absorbed  by  the  substance  on 
which  they  fall,  and  partly  thrown  off  again  by  reflection 
into  the  air.  The  effect  is  different  for  the  two  sub- 
stances, land  and  water.  Both  of  them  are  warmed  by 
the  addition  of  heat ;  but  the  rigidity  of  the  land  and  the 


30  ELEMENTARY   METEOROLOGY. 

mobility  of  the  water,  and  the  greater  heat  capacity  of  the 
latter,  make  the  separate  mention  of  each  necessary. 

The  Heat  received  by  the  Land  warms  it  very  much  at 
the  surface.  Part  of  the  heat  is  slowly  conducted  by  the 
ground  to  the  next  layers  of  earth  below ;  but  it  does  not 
penetrate  to  a  depth  of  many  feet,  because  the  land  is 
such  a  poor  conductor  of  heat.  Another  part  of  the  heat 
is  communicated  to  the  air  layer  nearest  the  ground,  and 
by  means  of  convective  air  currents  it  is  transported  to 
other  localities ;  the  change  being  accomplished  either  by 
the  transfer  of  the  heated  air  as  a  whole,  or  else  by  the 
heated  air  mingling  with  colder  air  and  imparting  heat 
to  it.  Another  portion  of  the  heat  of  the  surface  ground 
is  lost  through  outward  radiation  into  space.  Another 
part  of  the  heat  is  lost  from  the  land  by  reflection.  A 
small  portion  of  the  heat  passes  upward  from  layer  to 
layer  of  air  by  means  of  conduction  alone ;  but  this  is  an 
exceedingly  slow  process,  because  air  is  such  a  poor  con- 
ductor of  heat. 

The  Heat  received  by  the  Water  Surface  warms  it  but 
slightly ;  for  convection  currents  at  once  arise  where  the 
water  has  not  everywhere  the  same  warmth,  and  the  heat 
is  transferred  by  means  of  these  currents  to  cooler  por- 
tions of  the  water.  Heat  is  also  imparted  by  the  water 
surface  to  the  air,  and  is  radiated  outward  into  space  as 
from  a  land  surface,  although  more  feebly,  because  the 
surface  of  the  water  is  cooler;  but  the  amount  of  heat 
returned  to  the  air  by  reflection  from  the  water  surface  is 
a  very  important  part  of  the  whole  amount  communicated 
to  the  air.  Some  of  the  heat  received  at  the  surface  passes 
through  the  water  for  a  considerable  distance  by  continued 
direct  radiation,  and  is  gradually  absorbed  by  the  lower 
layers  of  the  water. 


TEMPERATURE. 


A  considerable  quantity  of  heat  is  also  used  up  in  the  process  of 
evaporation  from  the  water  surface. 

Principle  of  the  Thermometer.  —  In  general,  the  addi- 
tion of  heat  to  a  body  causes  it  to  expand,  and  the  loss 
of  heat  causes  it  to  contract.  Equal  increments  of  heat 
cause  an  equal  expansion,  and  equal  losses  of  heat  cause 
equal  contractions,  in  the  same  body.  The 
amount  of  heat  added  or  lost  can,  then,  be 
measured  by  the  expansion  or  contraction 
which  some  adopted  substance  undergoes 
when  subjected  to  these  changes. 

For  various  practical  reasons,  mercury 
is  the  substance  chosen  for  the  expansive 
or  thermometric  substance.  It  does  not 
congeal,  or  become  solid,  in  the  coldest 
weather  in  most  inhabited  countries,  and 
does  not  vaporize  in  the  hottest  weather ; 
and,  moreover,  its  expansion  and  contrac- 
tion, due  to  small  changes  in  the  amount  of 
heat  to  which  it  is  subjected,  are  relatively 
great  as  compared  with  those  of  other 
metals,  and  can  therefore  easily  be  seen.  FIG.  4.— THERMOMETER 

For  convenience  in  readily  determining  BuLB  AND  STEM' 
the  amount  the  mercury  has  expanded  or  contracted,  it 
is  placed  in  a  small  hollow  glass  bulb  with  a  connecting 
small  bored  tube  as  an  outlet  (Fig.  4).  The  bulb  is  filled 
with  mercury  at  some  rather  low  temperature,  and  any 
increase  of  heat  will  cause  the  mercury  to  flow  out  into  the 
capillary  tube.  The  larger  the  bulb  and  the  smaller  the 
bore  of  the  tube,  the  greater  will  be  the  amount  of  rise  of 
the  mercury  in  the  tube  for  a  given  increase  in  the  amount 
of  heat. 

Thermometer  Scales.  —  In  order  to  determine  just  how 


32  ELEMENTARY   METEOROLOGY. 

far  the  mercury  moves  in  the  thermometer  tube  when  it 
is  subjected  to  changes  of  temperature,  it  is  necessary  to 
have  some  kind  of  a  scale  for  such  measurements.  If  all 
thermometers  had  the  same  dimensions,  we  could  use  a 
scale  of  inches ;  but  this  cannot  ordinarily  be  used,  because 
thermometers  are  of  various  sizes. 

There  are  several  kinds  of  thermometer  scales  in  use, 
and  the  divisions  of  the  scale  are  called  degrees,  which 
are  of  the  same  length  in  each  individual  instrument. 
The  scale  divisions  are  marked  either  on  the  glass  tube 
or  stem  of  the  thermometer  or  on  a  metal  or  glass  strip 
placed  adjacent  to  it. 

Graduation  of  a  Thermometer.  —  There  are  two  temper- 
atures, which,  because  of  the  comparative  ease  with  which 
they  may  be  determined,  have  been  adopted  as  the  basis 
in  graduating  thermometer  scales.  These  are  the  temper- 
ature of  freezing  and  that  of  boiling  water.  These  tem- 
peratures are  called  the  fiducial  points  of  a  thermometer. 
In  marking  the  scale  of  a  thermometer,  the  position  of  the 
mercury  at  these  two  points  is  ascertained  by  holding  the 
instrument  first  in  freezing,  and  then  in  the  steam  from 
boiling  water  (under  normal  atmospheric  pressure  at  sea 
level).  The  graduation  is  then  completed  by  marking  off 
the  distance  between  these  points  in  equal  subdivisions. 
There  are  three  systems  of  thermometer  graduation  in 
use  in  different  part's  of  the  world,  —  the  Fahrenheit,  the 
Centigrade  or  Celsius,  and  the  Reaumur. 

In  the  Fahrenheit  thermometer  there  are  180  divisions 
or  degrees  between  the  fiducial  points ;  the  freezing  point 
is  marked  32°,  and  the  boiling  point  212°. 

In  the  Centigrade  thermometer  there  are  100  divisions 
or  degrees  between  the  fiducial  points ;  the  freezing  point 
is  marked  o°,  and  the  boiling  point  100°. 


TEMPERATURE. 


33 


In  the  Reaumur  thermometer  there  are  but  80  divisions 
or  degrees  between  the  fiducial  points ;  the  freezing  point 
is  marked  o°,  and  the  boiling  point  80°. 

The  graduation  of  standard  thermometers  is  carried  below 
the  freezing  and  above  the  boiling  points  to  some  distance, 
to  permit  of  the  determination  of  extreme  temperatures. 

Temperatures  below  the  o°  point  are  marked  with  a 
minus  sign;  those  above  are  considered  as  plus,  but  this 
sign  is  usually  omitted. 

The  Reaumur  scale  is  seldom  used  outside  of  Germany 
at  present,  and  is  therefore  not  further  described. 

Centigrade  degrees  are  converted  into  Fahrenheit  de- 
grees by  the  following  formula  :  - 

C°x  i.8  +  32°  =  F°. 

Fahrenheit  degrees  are  converted  into 
Centigrade  degrees  as  follows  :  — 

F°-32° 
1.8 

Fig.  5  represents  a  mercurial  thermom- 
eter with  an  attached  brass  scale  show- 
ing degrees  Fahrenheit.  The  temperature 
reading  is  70°. 

Effect  of  Change  of  Temperature  on  the 
Air. —  Starting  with  the  condition  of  the 
air  at  the  temperature  of  freezing  water 
(which,  as  we  have  seen,  is  constant,  and 
is  32°  on  Fahrenheit's  thermometer  scale, 
and  0°  on  the  Centigrade  scale),  the  in-  FIG.  5.- MERCURIAL 

THERMOMETER. 

crease  in  volume  within  the  limits  of  nat- 
ural temperatures  is  ^|T  of  itself  for  each  degree  of  tem- 
perature on  the  Fahrenheit  scale,  and  ^^  for  each  degree 
on  the  Centigrade  scale. 

WALDO    METEOR.  —  3 


34  ELEMENTARY  METEOROLOGY. 

Thus,  if  a  column  of  air  491  inches  long  at  a  temperature  of  32°  F. 
has  its  temperature  raised  i°  F.,  the  air  will  expand  to  492  inches  in 
length ;  and  if  the  temperature  is  lowered  i°  F.,  it  will  contract  to  490 
inches  in  length.  And  following  this  law,  if  the  temperature  were 
lowered  491°  F.,  then  the  length  of  the  column  of  air  would  be  reduced 
to  nothing  ;  according  to  which,  we  could  not  have  a  temperature  lower 
than  491°  F.  below  the  freezing  point  of  water,  and  this  would  be  nearly 
460°  F.  below  zero  of  the  Fahrenheit  scale. 

This  assumes  that  the  air  would  retain  its  gaseous  form  down  to  this 
low  temperature.  As  a  matter  of  fact,  all  of  the  constituents  of  the  air 
would  liquefy  and  solidify  before  such  temperatures  were  reached,  and 
so  would  cease  to  obey  the  law  of  expansion  and  contraction  of  gases, 
on  which  these  results  are  based.  Temperatures  of  about  —  375°  F. 
have  been  produced  by  artificial  means,  and  air  has  been  liquefied  and 
solidified  at  a  somewhat  higher  temperature  when  it  has  first  been 
made  very  dense  by  compression.  Of  the  constituents  of  the  air, 
oxygen  is  most  easily,  and  nitrogen  least  easily,  liquefied  and  solidified. 

Observations  of  Temperature.  — Temperature  observa- 
tions of  the  air  should  be  made  in  the  shade,  and  at  about 
six  to  ten  feet  above  grass-covered  earth ;  but  in  the  cities 
the  thermometers  are  frequently  exposed  from  upper 
windows  or  on  the  roofs  of  buildings.  Observations  of 
temperature  of  the  air  such  as  these,  have  been  made  at 
intervals,  during  longer  or  shorter  periods,  at  thousands 
of  places  scattered  over  the  earth's  surface.  Those  on  the 
land  have  generally  been  made  at  fixed  localities,  while 
those  on  the  ocean  have  been  made  mostly  on  moving 
vessels.  The  observed  air  temperatures  over  the  globe 
present  a  wide  variety  of  phases,  which  vary  with  the 
latitude,  altitude,  and  environment  of  the  various  localities 
considered.  Temperatures  also  vary  with  the  time ;  that 
is,  they  do  not  remain  constant  at  any  one  place. 

Fluctuations  of  Temperature.  —  The  sun  in  its  daily 
course,  due  to  the  rotation  of  the  earth,  gives  rise  to 
ditirnal  fluctuations  of  the  temperature ;  and  in  its  yearly 


TEMPERATURE.  35 

course,  due  to  the  revolution  of  the  earth  combined  with 
the  inclination  of  the  earth's  axis,  it  gives  rise  to  the 
gradual  changes  which  make  up  the  seasonal  fluctuations 
of  the  temperature.  There  are  also  long-period  fluctuations 
in  the  temperature,  which  extend  over  many  years. 

Seasonal  changes  in  temperatures  have  been  quite  care- 
fully studied  at  nearly  all  parts  of  the  surface  of  the  earth ; 
but  the  changes  during  the  day  have  been  carefully  studied 
at  a  few  places  only.  Long-period  fluctuations  have  been 
only  roughly  investigated. 

In  addition  to  these  regular  fluctuations  of  temperature, 
there  are  irregular  or  accidental  fluctuations,  due  to  causes 
which  will  be  mentioned  later. 

The  temperature  at  any  place  at  the  earth's  surface  de- 
pends principally  on  the  length  of  time  during  which  that 
point  is  exposed  to  direct  solar  radiation,  and  on  the  angle  at 
which  the  sun's  rays  reach  the  place.  Anything,  therefore, 
which  tends  to  interrupt  the  duration  of  solar  radiation,  or 
change  the  angle  of  inclination  of  the  solar  rays,  must 
cause  changes  in  the  temperature  of  the  point  reached 
by  those  rays. 

Temperature  Changes  during  One  Rotation  of  the  Earth.  — 
The  changes  of  temperature  during  the  diurnal  rotation 
of  the  earth  are  of  two  classes,  —  the  regular  or  periodic 
changes,  due  to  this  rotation,  which  vary  with  the  different 
conditions  under  which  the  solar  radiation  is  received  in 
different  parts  of  the  earth's  orbit ;  and  the  irregular  or 
accidental  changes,  due  to  the  oceanic  or  continental  loca- 
tion, the  varying  degrees  of  cloudiness,  and  the  movement 
of  air  masses  by  the  winds. 

These  irregular  changes  of  temperature  are  least  in 
the  equatorial  regions,  but  increase  towards  the  poles,  and 
in  middle  and  higher  latitudes  are  frequently  of  greater 


36  ELEMENTARY   METEOROLOGY. 

magnitude  than  the  regular  periodic  changes.  Their 
causes  are  mentioned  later  (p.  45  and  elsewhere). 

The  Regular  Diurnal  Change  of  the  Temperature  is  as 
follows :  — 

Beginning  at  sunrise,  there  is  a  gradual  increase  in  the 
temperature  with  the  increasing  altitude  of  the  sun,  until 
the  highest  or  maximum  temperature  is  reached  shortly 
after  the  time  when  the  sun  reaches  its  highest  altitude 
in  the  sky ;  then,  as  the  sun's  altitude  decreases,  the  tem- 
perature decreases,  until  (about)  the  time  when  the  sun 
rises  again  to  repeat  the  course  of  the  previous  day. 
There  is  thus  a  single  highest  and  a  single  lowest  tempera- 
ture during  the  24  hours. 

The  maximum  or  highest  temperature  of  the  24  hours 
is  reached  at  the  time,  after  the  sun  has  attained  its 
greatest  altitude,  when  the  amount  of  heat  lost  from  the 
earth  by  radiation  just  equals  the  amount  received  from 
the  sun.  This  occurs  at  various  times,  ranging  from  13 
hours  ( I  P.M.)  for  the  ocean  exposures  to  15  or  16  hours 
(3  or  4  P.M.)  over  the  continents. 

The  air  is  radiating  heat  into  space,  and  is  receiving 
heat  from  the  sun,  during  the  day.  When  the  temperature 
of  the  air  is  increasing,  it  shows  that  the  rate  at  which  the 
heat  is  received  from  the  sun  is  greater  than  that  at  which 
it  is  lost  by  radiation ;  and  it  will  keep  on  growing  warmer 
as  long  as  this  continues,  which  is  until  a  few  hours  after 
noon.  But  when  the  heat  received  and  that  given  off  by 
radiation  become  equal,  the  temperature  remains  stationary ; 
and  when  the  loss  of  heat  is  more  rapid  than  the  gain, 
there  is  a  fall  in  the  temperature. 

The  maximum  temperature  attained  over  the  land  is  greater  than 
that  on  the  ocean  ;  and  the  hour  when  the  amount  of  heat  received 
and  lost  is  equal  becomes  later  in  proportion  to  this  temperature,  which 


TEMPERATURE.  37 

accounts  tor  the  earlier  occurrence  of  the  maximum  over  the  ocean. 
The  hour  of  occurrence  does  not  vary  much  with  the  season  of  the 
year. 

The  degree  of  cloudiness  —  and  especially  during  the  hours  about 
noon  —  greatly  influences  the  maximum  temperature.  With  an  increase 
in  the  cloudiness,  there  is  a  decrease  in  the  maximum  temperature, 
because  the  solar  rays  cannot  reach  the  earth's  surface ;  and  there  is 
a  retardation  of  the  time  of  maximum,  because  the  radiation  of  heat 
from  the  earth  is  retarded. 

The  minimum  or  lowest  temperature  of  the  24  hours 
occurs  at,  or  just  a  few  minutes  before,  sunrise  over  the 
continents,  and  a  little  earlier  than  this  over  the  ocean. 

The  time  of  occurrence  is,  then,  at  about  6  hours  (6  A.M.)  at  the 
equator ;  and  with  increase  of  latitude,  it  becomes  earlier  in  the  summer 
half  year,  and  later  in  the  winter  half  year.  In  latitude  80°  the  min- 
imum occurs  at  about  2  A.M.  during  the  summer  half  year. 

The  minimum  temperatures  are  lower  over  continents  than  on  the 
oceans,  because  the  land  loses  its  heat  more  rapidly  than  the  water. 
Under  like  conditions  of  surroundings,  the  minimum  temperatures  get 
absolutely  lower  with  increasing  distance  from  the  equator  towards 
the  pole ;  but  they  do  not  descend  lower  below  the  average  temper- 
ature of  the  place  considered.  This  last  descent  is  greatest  at  the 
interior  of  deserts  in  the  equatorial  regions. 

The  effect  of  cloud  is  to  prevent  the  minimum  temperatures  from 
becoming  as  low  as  they  would  under  a  clear  sky,  for  the  radiation  of 
heat  from  the  earth  is  hindered. 

The  amplitude  of  regular  oscillation  of  the  diurnal  tem- 
perature (or  the  difference  between  the  extreme  maximum 
and  minimum  during  the  24  hours)  is  in  general  greatest 
at  the  equatorial  regions,  and  decreases  towards  the  poles, 
for  the  same  exposure ;  that  is,  over  the  land  or  over  the 
water.  The  amplitude  is  greatest  over  continents,  and 
least  over  oceans,  varying,  in  extreme  cases,  from  about 
32°  F.  for  continental  exposure  in  equatorial  regions,  to  2° 
or  3°  F.  in  polar  regions. 


ELEMENTARY   METEOROLOGY. 


The  amounts  of  amplitudes  follow  quite  closely  the  variation  in  the 
meridian  altitude  of  the  sun  and  the  relative  length  of  daytime  to  night- 
time ;  and  consequently  the  amplitudes  are  greatest  in  summer,  and 
least  in  winter. 

.  The  amplitudes  decrease  with  increase  of  altitude  above  the  ground, 
and  approach  in  magnitude  those  obtaining  for  an  oceanic  exposure. 

The  immediate  surroundings  of  a  locality  strongly  affect  the  amount 
of  the  amplitude  of  daily  temperature  changes.  If  the  amplitude  for  a 
plain  or  level  surface  is  taken  as  the  normal  condition,  then,  in  general, 
on  a  convex  surface  (a  hill  or  mountain)  the  amplitude  is  diminished, 
and  on  a  not  too  great  concave  surface  (a  valley)  the  amplitude  is 
increased.  In  the  case  of  the  hill,  the  heat  rays  are  dispersed,  while 
in  the  valley  they  are  collected. 

The  daily  amplitude  is  greatest  on  clear  days,  and  least  on  cloudy  days, 

because  the  maximum  tem- 
perature is  higher,  and  the 
minimum  lower,  on  the 
clear  day. 

Representation  of 
Diurnal  Temperatures. 
—  The  daily  course  or 
march  of  the  tempera- 
tures (and  of  the  other 
elements)  from  hour  to 
hour  is  represented  by 
meteorologists  in  sev- 
eral ways:  (i)  merely 
by  numbers,  which 
may  be  only  the  ac- 
tual temperatures,  or 
may  be  the  average 
temperature  for  the  day,  minus  the  individual  hourly  tem- 
peratures;  (2)  by  a  graphical  process  (Fig.  6);  and  (3)  by 
means  of  a  mathematical  formula,  called  Bessel's  Formula, 
which  expresses  this  curve  mathematically. 


_- 

—  . 

i 

>i 

g 

a 

, 

s" 

*" 

,~~ 

•^ 

1  — 

-«. 

Lk 

ec 

in 

/ 

V 

«r° 

""^ 

•-^ 

/J 

/ 

~~~*  -^ 

S 

K 

'  '/ 

I 

'€, 

[3 

^0 

J'o 

/• 

s^ 

/ 

^ 

/ 

Fa 

/ 

±5 

\ 

1 

\ 

1 

s^ 

X.. 

F0 

1 

40 

^ 

1 

6 

t. 

PC 

U 

s 

/ 

*•». 

*s 

1 

2A 

3* 

0*1 

1*1 

2*1 

3*1 

4*1 

6*1 

8*1 

0*2 

9*2 

I  2 

2*23*24 

FIG.  6.  —  DIURNAL  CHANGE  OF  AIR  TEMPERATURE. 


TEMPERATURE. 


39 


The  graphical  process  is  the  simplest  and  best  method  for  illustrating 
clearly  the  rise  and  fall  of  the  temperature  indicated  by  the  thermometer 
readings.  A  piece  of  paper  is  divided  into  small,  equal  squares  by 
vertical  and  horizontal  lines.  The  consecutive  vertical  lines  represent 
consecutive  hours,  commencing  either  at  oh  or  midnight,  or  at  ih;  the 
hours  are  marked  in  a  horizontal  row  below  the  diagram,  at  the  foot  of 
the  vertical  lines ;  and  the  consecutive  horizontal  lines  represent  con- 
secutive degrees  of  temperature,  commencing  at  some  degree  below 
the  lowest  temperature  which  is  to  be  represented,  and  the  degrees  are 
marked  in  a  vertical  column  to  the  left  of  the  diagram.  On  the  first  ver- 
tical line  put  a  dot  at  the  proper  point  for  showing  the  temperature  at  oh 
or  ih,  whichever  is  chosen  to  begin  with,  according  to  the  scale  of  de- 
grees marked  on  the  left  of  the  diagram ;  do  this  likewise  on  the  next 
vertical  line  for  the  temperature  at  the  next  hour ;  and  so  continue  for 
all  the  24  hours.  Then  join  these  points  by  a  succession  of  short 
lines,  and  the  resulting  curved  line  will  show  the  gradual  rise  and  fall, 
or  increase  and  decrease,  of  the  temperature  for  the  day. 

The  diurnal  changes  in  the  temperature,  taking  the  average  for  the 
year,  on  the  island  of  Key  West,  Fla.  (ocean  coast),  at  St.  Paul,  Minn, 
(continental),  at  Fort  Conger  in  the  Arctic  region,  and  also  over  the 
tropical  oceans,  are  shown  by  the  following  hourly  temperatures  in 
degrees  Fahrenheit:  — 


ih 

2h 

3h 

4h 

Sh 

6h 

7h 

8fc 

9h 

IQh 

nh 

I2h 

Tropical  Ocean, 
Key  West,  Fla. 
St.  Paul,  Minn. 
Fort  Conger, 

77.0 
74.1 

40.3 
—  i.i 

76.9 
74.0 
39-7 
—'•3 

76.8 
73-8 
39-1 

—  1.2 

76.7 

73-6 

38.5 
—  i.i 

76.7 
73-5 
37-9 
—  1.0 

76.8 

73-4 
37-6 
—0.7 

77-3 
74.0 

38.1 
—0.4 

77-9 
75-3 
38.9 

—0.2 

78.5 
76.4 
40.6 
O.I 

79.0 

77-3 
42.5 
0.6 

79-3 
78.1 

44-5 

1.0 

79-5 
78.3 
46.2 
1.2 

I3h 

I4h 

i5h 

I6h 

I7h 

i8h 

igh 

20^ 

2lh 

22h 

23h 

24.  h 

Tropical  Ocean, 

79-5 

79-3 

79.2 

79.0 

78.5 

78.2 

78.0 

77-8 

77-6 

77-5 

77.4 

77-3 

Key  West,  Fla. 

78.5 

78.7 

78.7 

78.2 

77.6 

76.7 

75-7 

75-3 

75-0 

74.8 

74.6 

74-4 

St.  Paul,  Minn. 

47-4 

484 

48.8 

48.9 

48.5 

47-6 

46.3 

45.° 

43-9 

42.8 

41.9 

41.0 

Fort  Conger, 

1.2 

i-3 

1.2 

1.0 

0.8 

0.6 

•03 

0.0 

—0.2 

-0.5 

—0.8 

—0.8 

The  heavy-faced  type  shows  the  maximum,  and  the  Italics  the  mini- 
mum, hourly  temperature. 

The  same  data,  except  for  Fort  Conger,  are  shown  by  the  graphical 
method  in  Fig.  6. 


40  ELEMENTARY  METEOROLOGY. 

The  Average  Daily  Temperature.  —  This  is  obtained  ac- 
curately by  taking  the  sum  of  the  temperatures  observed 
at  .each  of  the  24  hours,  and  dividing  it  by  24.  Gener- 
ally, however,  observations  are  made  but  two  or  three 
times  a  day;  and  it  is  customary  to  choose  such  hours  of 
the  day  for  times  of  observation  as  will  best  allow  the  aver- 
age temperature  to  be  computed  from  the  observations 
made  at  them.  Sometimes  two  separate  hours  are  chosen, 
but  more  frequently  three ;  and,  in  addition,  sometimes  the 
readings  of  maximum  and  minimum  recording  thermome- 
ters are  also  available. 

If  the  sum  of  the  temperature  observations  made  at  7  A.M., 
2  P.M.,  and  10  P.M.,  is  divided  by  3,  very  nearly  the  true 
average  daily  temperature  will  be  obtained ;  and  special 
combinations  of  other  hours  will  give  nearly  as  accurate 
results.  The  average  of  the  observations  at  8  A.M.  and 
8  P.M.,  as  observed  by  the  United  States  Weather  Bureau, 
gives,  within  a  fraction  of  a  degree,  the  average  tempera- 
ture for  the  day. 

There  must  be  two  times  during  the  daily  march  of  the  temperature 
when  the  average  temperature  of  the  day  is  reached,  —  one  during  the 
time  of  increase  of  heat,  and  the  other  during  the  time  of  decrease, 
—  because  this  average  lies  between  the  maximum  and  minimum  tem- 
peratures. The  average  temperature  for  the  day  occurring  during  the 
morning  increase  of  heat  is  reached  at  about  8h  over  a  water  surface, 
and  at  about  9h  over  a  land  surface ;  and  during  the  evening  decrease 
of  heat,  it  is  reached  often  as  late  as  4  hours  after  sunset  in  winter,  and 
at  about  sunset  in  summer,  over  the  land  surface,  while  over  the  water 
surface  it  occurs  at  about  an  hour  earlier  than  over  the  land. 

Temperature  Changes  during  the  Revolution  of  the  Earth. 

-  The  inclination  of  the  earth's  axis  causes  a  difference  in 
the  length  of  the  days  and  nights,  and  a  variation  in  the 
intensity  of  the  solar  rays  at  points  on  the  surface,  when 
the  earth  is  in  different  parts  of  its  orbit. 


TEMPERATURE.  41 

The  meteorological  seasons  (winter,  spring,  summer, 
and  autumn),  while  each  embraces  three  months  in  our 
temperate  latitudes,  are  quite  arbitrarily  fixed  when  we 
consider  the  earth's  surface  as  a  whole ;  for  they  neither 
occur  during  the  same  months  in  the  two  hemispheres,  nor 
do  they  have  the  same  duration  in  different  latitudes. 
With  us  the  coldest  month  (January)  is  taken  as  the  mid- 
winter month,  and  the  warmest  month  (July)  as  the  mid- 
summer month ;  and  the  mid-spring  month  (April)  and  the 
mid-autumn  month  (October)  come  midway  between  these. 
It  is  only  in  middle  latitudes  that  the  four  seasons  are  all 
well  marked  and  of  approximately  equal  length.  In  lower 
latitudes  the  change  from  winter  to  summer  is  but  slight, 
and  the  transition  occurs  so  gradually  as  to  be  almost 
imperceptible  so  far  as  the  temperature  is  concerned.  In 
the  very  high  latitudes,  on  the  contrary,  the  change  from 
summer  to  winter  and  from  winter  to  summer  is  very 
abrupt,  and  there  is  really  only  a  winter  and  a  summer 
season. 

The  seasons,  except  for  local  purposes,  are  to  a  great 
extent  losing  their  former  importance  in  meteorological  re- 
ports, because  of  late  years  nearly  all  data  are  given  for  the 
separate  months  and  for  the  calendar  year,  in  order  that 
the  observations  made  all  over  the  globe  may  be  readily 
compared.  For  agricultural  purposes  the  average  meteor- 
ological conditions  during  the  seasons  are  important. 

The  times  of  the  seasons  are  reversed  in  the  northern 
and  southern  hemispheres.  When  it  is  winter  in  the 
northern  hemisphere,  it  is  summer  in  the  southern  hemi- 
sphere ;  and  when  it  is  spring  in  the  northern  hemisphere, 
it  is  autumn  in  the  southern  hemisphere.  In  middle  lati- 
tudes, where  three  months  are  given  to  each  season,  they 
are  as  follows  :  — 


42  ELEMENTARY   METEOROLOGY. 

Months.  Dec.  Jan.  Feb.     Mar.  Apr.  May.   June.  July.  Aug.    Sept.  Oct.  Nov. 

Seasons  in  the 

northern    hemi-  [      Winter.  Spring.  Summer.  Fall, 

sphere. 

Seasons  in  the 

southern     hemi-  j-      Summer.  Fall.  Winter.  Spring, 

sphere. 

In   Fig.   2  these  ) 

periods  B.  C.  D.  A. 

are  shown  at     J 

Gradual  Change  of  Seasons.  —  The  change  in  the  angle 
of  inclination  of  the  solar  rays,  and  in  the  length  of  days, 
from  day  to  day  during  the  progress  of  seasons,  is  very 
slight ;  and  except  near  the  poles  the  resulting  change  in 
the  temperatures  of  places  on  the  earth's  surface  is  cor- 
respondingly small.  We  have,  then,  slow  temperature 
changes  during  each  season,  and  but  a  gradual  passing 
from  one  season  to  the  next. 

In  following  out  the  changes  in  temperature  due  to  the  revolution  of 
the  earth  around  the  sun,  we  have,  then,  naturally  to  turn  to  the  series 
of  average  temperatures  for  each  day  for  successive  days  during  the 
year.  Accidental  causes  which  give  rise  to  variable  temperature  condi- 
tions from  day  to  day,  so  disguise  this  slow  gradual  change,  that  we 
can  realize  its  occurrence  only  after  the  lapse  of  a  number  of  days. 

The  Yearly  Course  or  March  of  the  Temperature.  —  This 
is  represented  by  the  series  of  average  temperatures  for 
equal  fractions  of  a  year,  usually  for  the  months ;  but  in 
some  cases  five-day  averages  are  used.  Single  daily  aver- 
ages are  not  used,  because  even  a  century  of  observations 
would  not  give  the  average  daily  temperature  with  suffi- 
cient accuracy  to  remove  all  accidental  irregularities  which 
occur  in  them. 

There  exists  during  the  year  (away  from  the  equatorial 


TEMPERATURE.  43 

region)  but  a  single  maximum  and  a  single  minimum,  as 
was^seen  to  be  the  case  for  the  daily  temperature.  In  the 
northern  hemisphere  the  warmest  month  is  July,  and 
the  coldest  is  January,  which  is  about  a  month  later  than 
the  times  when  the  sun  reaches  the  highest  and  lowest 
altitudes  respectively.  On  and  near  the  oceans  the  greatest 
retardation  takes  place.  In  the  southern  hemisphere  the 
months  of  maximum  and  minimum  temperatures  just -stated 
are  reversed.  It  thus  occurs  that  in  the  region  about  the 
equator,  the  maximum  temperatures  occur  at  the  time 
when  higher  latitudes  are  enjoying  medium  temperatures 
(spring  and  fall).  Two  maxima  and  two  minima  thus  exist 
for  the  equatorial  region,  but  the  amplitudes  are  compara- 
tively slight. 

The  reason  that  the  highest  and  lowest  monthly  temperatures  occur 
later  than  the  times  of  highest  and  lowest  meridian  altitude  of  the  sun, 
is  similar  to  that  for  the  retardation  in  these  phases  in  the  diurnal 
changes  of  temperature.  When  the  whole  amount  of  heat  received 
from  the  sun  during  the  24  hours  of  the  day  just  equals  the  amount 
lost  from  the  earth  by  radiation,  then  the  average  daily  temperature 
remains  the  same.  But  as  the  sun  gets  higher  in  the  heavens  in  the 
spring,  the  amount  of  heat  received  from  it  exceeds  more  and  more  the 
amount  lost  by  radiation,  and  the  days  grow  warmer.  When  the  sun 
recedes  from  the  tropics  in  summer,  the  amount  of  heat  received  still 
exceeds  for  another  month  or  so  the  amount  lost :  consequently  the 
average  temperature  still  increases  during  this  time,  and  continues 
to  until  the  sun  has  moved  so  far  equatorward  that  the  amounts  of 
heat  received  and  lost  are  equal.  The  average  temperature  also  con- 
tinues to  decrease  during  some  weeks  after  the  sun  has  reached  its 
lowest  meridian  altitude,  for  the  amount  of  heat  lost  by  radiation  still 
exceeds  the  amount  received  from  the  sun  until  the  sun's  altitude  has  con- 
siderably increased.  In  our  latitude  the  two  become  equal  in  January. 

The  amplitude  of  the  yearly  temperature  period  is 
usually  understood  to  be  the  difference  between  the 


44 


ELEMENTARY   METEOROLOGY. 


highest  (maximum)  and  the  lowest  (minimum)  monthly 
averages.  This  amplitude,  for  the  same  surroundings, 
increases  from  the  equator  poleward.  The  land  and  water 
distribution  very  powerfully  influences  the  yearly  ampli- 
tudes, which  are  least  over  the  oceans ;  so  that  a  very  dry 
continental  location  near  the  equator  may  have  as  great  an 
amplitude  as  a  moist  oceanic  location  far  to  the  poleward. 

For  instance,  the  yearly  amplitude  at  Biskra  in  the  Sahara  (conti- 
nental) is  39.8°  F.,  and  at  Madeira  (oceanic)  it  is  but  10.8°  F.  At 
Vardo  (oceanic),  northwestern  Europe,  the  amplitude  is  25.9°  F.,  and 
at  Werchojansk  (continental),  Siberia,  it  is  1 17.9°  F.  At  this  last  place 
there  has  been  observed  a  temperature  of  —90°  F.  in  winter,  and  of 
+  86°  F.  in  summer,  thus  making  an  absolute  extreme  amplitude  of 
about  1 80°  F.  in  the  year. 

The  temperature  amplitudes  for  the  year  in  our  middle 
latitudes  increase  along  the  same  parallel  from  the  western 
coast  towards  the  middle  and  eastern  parts  of  the  conti- 
nents, and  then  decrease  again  towards  the  eastern  coasts. 
On  the  eastern  coast  the  amplitudes  are  greater  than  on 
the  western  coast,  on  account  of  the  great  influences  of 
the  prevailing  winds. 

With  increase  of  altitude,  the  yearly  amplitude  decreases, 
and  more  nearly  approaches  that  for  a  marine  exposure  at 
the  same  latitude. 

Below  are  given,  in  degrees  Fahrenheit,  average  monthly  temperatures 
for  Key  West,  Fla.,  St.  Paul,  Minn.,  and  Fort  Conger  in  the  Arctic  region. 


Jan. 

Feb. 

Mar. 

April 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Key   West 
(Seashore) 

69.7 

71.4 

73-2 

76.6 

79-8 

83.0 

84.1 

84.3 

82.2 

78.8 

74-3 

70.0 

St.    Paul 
(Continental) 

77.7 

17-5 

28.2 

44-9 

58.8 

67.2 

71.9 

6y.2 

58-7 

47.0 

31.0 

18.1 

Fort  Conger    .  .  . 

-38.2 

-40.1 

-28.1 

-13-6 

14.1 

32.6 

37-i 

33-8 

15-8 

-8.9 

-23.6 

-28.1 

The  same,  except  for  Fort  Conger,  are  shown  graphically  in  Fig.  7. 


TEMPERATURE. 


45 


Excessive  Range  of  Monthly  Temperatures.  —  Excessive 
amplitudes  of  temperature  are  due  to  the  extremely  low 
minimum  temperatures,  and  not  to  unusually  high  maxi- 
mum temperatures ;  the  minima  going  lower  than  usual, 
and  not  the  maxima  going  higher. 

This  is  realized  by  considering  the  fact  that 
we  should  call  a  day  warm  when  the  thermom- 
eter reaches  85°  F.,  but  very  hot  if  it  reaches 
100°  F.,  the  difference  being  but  15°  F.  While 
30°  F.  would  be  considered  quite  cold,  yet  a 
fall  of  the  temperature  to  20°  F.  below  zero  is 
not  an  unusual  experience  in  the  northern  part 
of  the  United  States ;  and  this  would  mean  a 
lowering  of  the  temperature  by  50°,  which  is 
over  three  times  as  great  as  the  increase  of 
15°  F.  just  mentioned. 

The  highest  average  monthly  temperature 
ever  observed  is  that  of  102°  F.  for  July,  at 
Death  Valley,  Cal. ;  and  the  lowest  is  -66°  F. 
for  January,  at  Werchojansk,  Siberia.  Proba- 
bly the  temperatures  in  some  parts  of  the 
Desert  of  Sahara  are  greater  than  those  for 
California,  but  we  have  little  accurate  knowl- 
edge of  the  temperatures  of  that  African  re- 
gion. 

The  average  temperature  for  the  year 
is  obtained  by  taking  the  sum  of  the 
average  temperatures  for  all  the 
months,  and  dividing  it  by  the  number  of  months.  Since 
monthly  temperatures  vary,  so  the  yearly  average  temper- 
ature will  vary,  but  not  so  much  as  the  monthly  temper- 
atures ;  and  these  last,  in  turn,  are  less  variable  than  the 
daily  temperatures. 

Irregular  Changes  of  Temperature  occur,  which  depend  on 
the  degree  of  cloudiness,  effects  of  radiation,  and  on  the 


VJFMAMJJAX  0  X  D 

FIG.  7.  —  ANNUAL  CHANGE 

OF  AIR  TEMPERATURES. 


46 


ELEMENTARY   METEOROLOGY. 


movement  of  air  masses.  These  temperature  changes  are 
local,  but  may  be  of  wide  extent.  When  irregular  rapid 
fluctuations  of  the  temperature  take  place,  the  distribution 
of  the  air  temperature  near  the  earth's  surface  is  usually 
as  follows :  — 

There  is  a  central,  more  or  less  limited,  area  of  high  or 
low  temperature,  from  which  the  temperature  irregularly 
but  gradually  decreases  or  increases,  as  the  case  may  be,  in 
all  directions. 


500  Miles 
FIG.  8.  —  AREAS  OF  Low  AND  HIGH  TEMPERATURE. 


1000  Miles. 


These  areas  are  shown  in  a  general  way  in  the  accompanying  dia- 
grams (Fig.  8),  in  which  the  curved  lines  inclose  limited  areas  on 
the  earth's  surface.  In  the  figure  on  the  left,  the  air  in  the  central  area 
has  a  temperature  of  —20°  F.  or  less ;  around  this  is  a  zone  in  which 
the  temperature  is  from  —20°  to  — 10°  F.,  and  around  this  are  others 
in  which  the  temperatures  are  from  —  10°  to  o°  and  from  o°  to  -f  10°  F. 
respectively.  In  the  figure  on  the  right,  the  central  area  has  a  tempera- 
ture of  80°  F.  or  over,  and  within  the  successive  surrounding  rings  the 
temperature  is  from  -f8o°  to  +70°  F.,  from  +70°  to  +60°  F.,  etc. 
Many  times  these  areas  do  not  appear  closed  on  all  sides,  and  this 
regularity  of  increase  or  decrease  of  temperature  does  not  take  place 
on  the  open  side  within  the  limits  of  the  region  considered.  These 


TEMPERATURE.  47 

areas  of  abnormally  cold  or  warm  air  move  over  the  earth's  surface 
in  regular  tracks,  and  this  causes  the  temperature  irregularities  for 
particular  places  to  have  quite  constant  average  values,  depending  on 
the  location  of  these  places  with  reference  to  the  tracks. 

Sometimes  these  areas  extend  over  many  thousands  of  square  miles, 
the  inner  region  of  greatest  or  least  temperatures  sometimes  covering 
more  than  one  of  our  States. 


The  Temperature  Anomaly.  —  If  the  average  tempera- 
ture for  the  same  month  during  many  successive  years  is 
obtained,  and  the  difference  between  this  average  and  the 
monthly  temperature  for  each  year  is  taken,  the  mean  of 
these  differences  will  be  the  mean  temperatitre  anomaly  for 
that  month.  Temperature  anomalies  are  the  least  in  lower 
latitudes,  and  are  greater  at  the  interior  of  continents  than 
on  the  oceans.  They  are  the  greatest  in  winter,  and  least 
in  midsummer.  The  mean  anomalies  are  relatively  quite 
constant  for  adjacent  places. 

Variability  of  Temperature.  —  The  variability  of  temper- 
ature from  day  to  day  is  the  difference  between  the  aver- 
age temperatures  for  successive  days.  The  average  of  such 
temperature  differences  is  usually  computed  for  each 
month.  Temperature  variability  is  least  in  the  tropics,  and 
increases  with  the  latitude  up  to  about  latitude  50°,  whence 
it  decreases  again  toward  the  poles.  It  increases  from  the 
sea  towards  the  interior  of  continents,  and  also  with  the  alti- 
tude on  the  average  for  the  year ;  and  up  to  a  certain  limit 
it  decreases  in  summer  and  increases  in  winter  with  the 
altitude.  It  is  greater  on  the  east  than  on  the  west  coast  in 
our  latitudes,  and  is  greatest  in  winter  and  least  in  summer. 

Concerning  the  extreme  variability  of  mean  daily  tem- 
perature from  day  to  day,  it  may  be  said  that  changes 
of  4°  F.  occur  in  all  parts  of  the  world,  of  7°  F.  in  most 
parts,  and  of  10°  F.  in  but  few  parts.  In  the  interior  of 


48  ELEMENTARY   METEOROLOGY. 

continents,  as,  for  instance,  North  America  and  Asia, 
individual  changes  of  even  45°  F.  occur,  but  at  rare 
intervals.  The  amount  and  frequency  of  the  individual 
temperature  changes  increase  towards  the  interior  of  a 
continent,  but  in  our  middle  latitudes  are  greater  on  the 
eastern  coast  of  the  continent  than  on  the  western. 

Average  Temperatures  for  Adjacent  Regions.  —  The  difference  in  the 
temperature  for  adjacent  places  is  a  much  more  constant  quantity  than 
are  the  absolute  temperatures  themselves.  For  instance,  suppose  that 
the  temperature  has  been  observed  at  two  neighboring  places,  —  at  the 
one  for  the  last  100  years,  and  at  the  other  for  only  the  last  10  years,  — 
then  the  best  way  of  obtaining  the  true  average  temperature  of  the 
second  place  is  to  find  the  average  temperature  at  both  places  during 
the  same  10  years,  and  take  the  difference  between  the  two ;  and  this 
difference  applied  to  the  average  for  the  one  place  for  100  years 
will  give  the  average  temperature  for  the  second  place  much  more 
accurately  than  it  has  been  determined  from  the  10  years1  observations 
alone. 

Long-period  Temperature  Oscillations  are  shown  by  direct 
observation  of  thermometers,  and  by  several  indirect  meth- 
ods, such  as  the  dates  of  the  harvests,  the  opening  and 
closing  of  navigation  by  ice,  the  relative  severity  of  winters, 
and  other  methods.  Observations  (mostly  European) 
show  a  periodic  fluctuation  of  temperature  during  a  period 
of  about  35  years.  During  the  past  century  the  years 
1791  to  1805  were  relatively  warm;  1806  to  1820,  cold; 
1821  to  1835,  warm;  1836  to  1850,  cold;  1851  to  1870, 
warm;  and  1871  to  1885,  cold.  The  average  fluctuation 
amounts  to  about  2°  F.  during  these  periods. 

Change  of  Temperature  with  the  Altitude  above  the  Sur- 
face of  the  Earth.  —  The  normal  condition  of  the  tempera- 
ture of  the  air  is  a  decrease  with  the  altitude  above  the 
earth's  surface.  This  decrease  is  not  the  same  in  amount 


TEMPERATURE.  49 

at  all  places  on  the  earth,  nor  is  it  constant  at  any  one 
place.  In  fact,  it  frequently  occurs  that  for  a  short  time, 
and  for  a  short  distance  upward,  there  is  an  increase  of 
temperature  with  the  altitude.  The  observed  decrease 
of  temperature  with  the  altitude  is,  on  the  average,  about 
i°  F.  for  330  feetjj  but  it  is  more  rapid  in  summer  than  in 
winter,  and  it  is  more  rapid  in  the  lower  air  layers  than 
in  the  higher.  In  the  winter  and  in  the  nighttime,  in  clear 
weather,  the  air  in  the  valleys  is  cooler  than  that  on  the 
low  hilltops ;  and  in  summer  and  in  the  daytime,  the  air  in 
the  low  valleys  is  warmer. 

Observations  have  been  made  on  mountains  and  in  balloons  with 
the  view  of  determining  accurately  the  law  of  the  decrease  in  tempera- 
ture with  the  altitude ;  but  such  a  great  variety  of  results  have  been 
obtained,  that  the  matter  is  at  present  by  no  means  definitely  settled. 

The  laws  of  thermodynamics  and  adiabatic 1  cooling  show  that  for 
dry  air  there  will  be  a  decrease  of  temperature  of  about  i°  F.  for  183 
feet  of  increase  in  altitude,  and  a  corresponding  increase  of  temperature 
for  a  like  decrease  in  altitude.  The  usually  observed  decrease  of  tem- 
perature with  the  altitude,  as  has  been  stated,  falls  short  of  this  amount. 
The  outer  limit  of  the  decrease  of  temperature  in  the  atmosphere,  or  the 

1  By  adiabatic  heating  or  cooling  of  a  mass  of  air  is  meant  a  change  in  its 
temperature  brought  about  by  a  change  in  its  density,  and  without  the  addition 
of  the  heat  from  or  the  loss  to  outside  sources.  Dry  air  will  cool  adiabatically 
about  i°  F.  if  its  density  is  decreased  by  a  change  in  pressure  equal  to  that 
involved  in  a  change  in  altitude  of  183  feet.  Hence,  as  a  result  of  the  de- 
crease in  the  pressure  and  density  of  dry  air  with  altitude,  there  will  be  a 
corresponding  decrease  in  the  temperature  of  i°  F.  for  each  183  feet  of 
altitude.  When  the  pressure  upon  a  body  of  air  is  decreased,  the  air  expands. 
To  effect  this  expansion,  part  of  the  heat  energy  of  the  air  is  utilized,  and 
becomes  insensible  as  heat;  hence  the  air  becomes  cooler,  or  its  temperature 
falls,  as  a  result  of  its  expansion.  On  the  other  hand,  when  the  pressure  on  a 
body  of  air  increases,  the  air  is  compressed  into  less  space;  and  part  of  the 
energy  which  was  employed  in  maintaining  its  former  bulk,  being  relieved  of 
this  duty,  appears  as  sensible  heat;  wherefore  the  air  becomes  warmer,  or  its 
temperature  rises,  as  it  grows  denser  under  compression. 

WALDO    METEOR.  —  4 


5O  ELEMENTARY   METEOROLOGY. 

temperature  at  points  above  the  earth's  surface  where  the  air  is  very  rare, 
is  not  known  with  any  degree  of  certainty  ;  but  it  has  been  thought  to  be 
about  —45.5°  F.,  judging  from  the  observed  decrease  of  temperature 
up  as  high  as  ten  or  twelve  thousand  feet.  A  temperature  of  —  54°  F. 
has,  however,  been  observed  in  a  balloon  at  an  altitude  of  31,500  feet. 

This  temperature  would  be  the  same  above  the  pole  and  above  the 
equator.  It  is  seen,  then,  that  in  Siberia,  with  an  average  tempera- 
ture for  January  of  —  58°  F.,  there  must  be  an  increase  in  the  temperature 
with  the  altitude  for  a  relatively  long  time  ;  but  it  is  probable  that  this 
increase  in  such  cases  extends  upwards  for  a  short  distance,  and  that 
it  reaches  a  limit  at  a  not  great  altitude,  from  which  point  there  is  a 
decrease  farther  upwards ;  that  is,  there  is  a  relatively  warmer  mass  of 
air  somewhere  between  the  very  cold  air  layer  at  the  earth's  surface  and 
the  very  cold  outer  limit  of  the  atmosphere. 

The  rate  of  decrease  varies  for  different  months  of  the  year,  and  the 
amount  of  this  variation  is  much  greater  for  inland  mountain  regions 
than  for  those  near  the  ocean.  The  maximum  rate  of  decrease  is  about 
i°  F.  for  200  feet  in  most  cases ;  and  the  great  variation  which  occurs 
for  individual  cases  is  due  to  inequalities  in  the  minimum  rates,  which 
appear  to  increase  with  the  latitude.  The  average  rate  of  decrease  for 
the  year,  while  varying  from  i°  F,  per  370  feet  to  i°  F.  per  220  feet,  is 
probably  about  i°  F.  in  330  feet  altitude. 

The  rate  of  temperature  decrease  with  the  altitude  also  varies  from 
hour  to  hour  during  the  day,  and  moreover  is  greatly  influenced  by  the 
degree  of  cloudiness. 

Near  the  ground  at  midday  in  clear  weather  the  decrease  of  tem- 
perature with  altitude  is  much  more  rapid  than  in  cloudy  weather ;  but 
at  evening  the  decrease  in  cloudy  weather  is  greatest. 

At  an  altitude  of  several  hundred  feet  above  the  ground  the  midday 
decrease  in  clear  weather  is  not  much  greater  than  for  cloudy  weather, 
and  may  even  become  equal  to  it ;  but  in  the  evening  the  decrease 
is  less  during  clear  weather  than  for  cloudy  weather.  The  decrease  of 
temperature  in  cloudy  weather  is,  then,  quite  constant  for  both  day  and 
night  above  the  very  lowest  air  layers ;  but  in  clear  weather  it  varies 
greatly.  The  cases  of  inversions  of  temperatures,  or  increase  of  tem- 
perature with  altitude,  occur  in  clear  weather. 

Geographical  Distribution  of  Air  Temperatures  over  the 
Earth's  Surface.  —  If  the  earth  had  a  homogeneous  surface 


TEMPERATURE.  5 1 

of  either  land  or  water,  the  average  temperature  for  the 
year  would  decrease  from  a  maximum  at  the  equator  to  a 
minimum  at  the  poles,  and  the  average  temperature  along 
any  parallel  could  be  considered  as  dependent  on  the  lati- 
tude alone. 

If  the  surface  were  all  land,  it  would  have  a  higher 
temperature  than  if  it  were  all  water,  in  the  equatorial 
regions,  because  land  is  more  easily  warmed  than  water; 
but  in  the  polar  regions  the  water  surface  would  have  the 
higher  temperature,  because  the  land  loses  its  heat  by 
radiation  faster  than  the  water.  At  some  middle  latitude 
the  temperatures  for  the  two  surfaces  would  be  equal. 

If  we  compare  the  temperatures  for  an  ideal  land-cov- 
ered with  that  of  a  water-covered  earth,  then  at  the  equa- 
tor the  temperature  of  the  land  would  be  115°  F.,  and  of 
the  water  nearly  72°  F.  ;  and  the  difference  between  the 
equatorial  and  polar  temperatures  would  be,  for  the  land 
about  135°  F.,  and  for  the  water  about  54°  F. 

In  the  actual  case  of  the  unequal  and  irregular  distribu- 
tion of  land  and  water  on  the  surface  of  the  earth,  however, 
the  average  temperature  along  any  parallel  is  dependent 
on  this  distribution  as  well  as  on  the  latitude.  There  are 
two  other  causes  which  are  also  effective,  but  to  a  much 
less  degree.  These  are 'the  transference  of  heat  toward 
the  poles,  and  cold  toward  the  equator,  by  means  of  ocean 
currents;  and  the  transference  of  heated  or  cooled  air  by 
the  prevailing  winds.  The  effects  of  these,  however,  are 
practically  neutralized  over  the  entire  circumference  of  the 
earth,  because  where  one  part  is  rendered  cooler  by  a  cold 
current,  another  is  made  warmer  by  a  warm  current. 

Isothermal  Charts.  —  An  isothermal  line,  or  isotherm,  is 
a  line  every  point  of  which  has  the  same  temperature. 
An  isothermal  surface  is  a  surface  every  point  of  which 


TEMPERATURE.  53 

has  the  same  temperature.  When  the  average  tempera- 
tures for  the  year  at  different  places  are  written  down  on 
a  map  at  those  places,  and  the  isothermal  lines  are  drawn 
on  the  map,  then  we  have  a  chart  of  the  isothermal  lines 
for  the  year  for  the  region  covered.  These  isothermal 
lines  show  where  the  isothermal  surfaces  of  the  air,  which 
are  inclined  downwards  from  the  equatorial  towards  the 
polar  regions,  intersect  the  level  of  the  surface  of  the 
earth.  Charts  of  this  kind  have  been  made  for  the  whole 
earth,  except  for  the  polar  regions,  for  which  we  have  few 
observations.  Similar  charts  have  been  constructed,  show- 
ing the  average  temperatures  for  each  month  of  the  year. 
Such  charts  for  the  year  (Fig.  9),  for  the  midwinter  month, 
January  (Fig.  10),  and  for  the  midsummer  month,  July 
( Fig.  1 1 ),  are  given  here. 

Courses  of  the  Isotherms.  —  As  was  to  have  been  ex- 
pected from  what  has  just  been  said,  the  isothermal  lines 
for  the  year  (Fig.  9)  do  not  follow  the  parallels  of  latitude 
around  the  globe.  These  irregularities  are  due  to  three 
main  causes  :  —  , 

1.  The  unequal  distribution  of  the  land  and  water  sur- 
faces, and  the  excessive  heating  of  the  land  in  summer 
and  cooling  in  winter,  as  compared  with  a  water  surface. 

The  irregularities  are  much  more  marked  in  the  northern  than  in 
the  southern  hemisphere,  because  in  the  former  the  ratio  of  land  to 
water  is  the  greater.  The  excessive  cooling  of  the  interior  of  the  con- 
tinents in  winter  causes  the  isothermal  lines  to  make  there  a  bend 
equatorward  at  that  season,  while  in  summer  the  excessive  heating  of 
the  land  causes  the  isotherms  to  make  a  poleward  bend. 

2.  The  transference    of   the  equatorial  heat  poleward, 
and  the  polar  cold  equatorward,  by  means  of  the  oceanic 
currents ;  and  this  heat  and  cold  are  communicated  to  the 
air. 


54  ELEMENTARY  METEOROLOGY. 

The  great  oceanic  currents  give  a  greater  warmth  to  the  waters  in 
the  western  parts  than  to  those  in  the  eastern  parts  of  the  oceans  up  to 
about  latitude  40°,  where  the  warm  current  coming  from  the  equator, 
along  the  eastern  coast  of  the  continent,  crosses  the  ocean ;  and,  divid- 
ing as  it  approaches  the  western  continental  coast,  part  of  it  flows  pole- 
ward, taking  abnormally  warm  water  with  it,  while  part  of  it  flows 
equatorward,  taking  with  it  colder  water  than  was  to  be  found  at  the 
same  latitude  on  the  western  side  of  the  ocean.  On  the  eastern  conti- 
nental coast  to  the  poleward  of  latitude  40°,  where  the  warm  current 
from  the  equator  crosses  the  ocean,  there  is  a  cold  current  coming  from 
the  polar  regions,  which  makes  the  coast  waters  abnormally  cold. 

3.  The  transference  of  heated  and  cooled  air  by  the 
prevailing  winds. 

In  the  middle  latitudes  of  the  two  hemispheres  the  winds  from  the 
west  blow  in  winter  the  colder  air  from  the  continents  on  to  the  west- 
ern parts  of  the  ocean ;  and  the  isothermal  lines  crossing  the  oceans 
from  west  to  east  are  inclined  poleward  (that  is,  in  a  northeasterly 
direction  in  the  northern  hemisphere),  and  separate  farther  and  farther 
apart  in  their  progress.  The  isothermal  lines  crossing  the  continents 
are  inclined  toward  the  equator  (that  is,  in  a  southeasterly  direction 
in  the  northern  hemisphere),  and  converge  more  and  more  in  their 
progress,  because  the  prevailing  winds  are  from  the  west,  and  in  winter 
blow  the  warmer  oceanic  air  on  to  the  western  coasts  of  the  continents, 
and  the  cooler  continental  air  on  to  the  eastern  coast  regions.  In  the 
summer  the  winds  from  the  west  blow  the  cooler  oceanic  air  on  to  the 
western  coasts  of  the  continents,  while  the  same  winds  blow  the  heated 
continental  air  on  to  the  eastern  coasts.  Thus  the  western  coasts  are 
kept  cooler,  and  the  eastern  become  warmer. 

In  the  equatorial  zone  the  winds  blow  mostly  from  the  east,  and  the 
conditions  depending  on  the  general  direction  of  the  wind  are  in  gen- 
eral the  reverse  of  those  in  middle  latitudes. 

A  more  detailed  statement  of  these  reasons  is  deferred 
to  the  chapter  on  climates. 

Normal  Average  Temperatures.  —  The  average  temper- 
atures for  the  different  degrees  of  latitude  have  been  derived 


(55) 


ELEMENTARY    METEOROLOGY. 


by  taking  the  average  of  the  temperatures  at  various 
points  along  individual  parallels.  The  following  table 
shows  these  average  temperatures  for  the  year,  for  Jan- 
uary, and  for  July,  along  each  10°  of  latitude,  both  north 
and  south  of  the  equator :  — 


LATITUDE. 

JANUARY. 

JULY. 

YEAR. 

North  80° 

F. 
—  3O  Q° 

F. 

-4-  1.2  1° 

F. 

4-    r  6: 

North  70°      
North  60°      

-15.7° 
+      7  Qr/ 

+  44.I° 

t;6o° 

13-7° 
29  8° 

North  50°                     .     .     . 

10  Q° 

64.6° 

4.2  C0 

North  40° 

42  Q° 

7S  4° 

**o 

C7  1° 

North  30°      
North  20°      

59-5° 

71.7° 

/>*» 

8  1.0° 
82.4." 

J/'1 
68.4° 
76  Q' 

North  10°                      ... 

78.  S° 

807° 

80  8: 

Equator    o° 

800° 

78  2° 

7Q  Q° 

South  10° 

800° 

74  Q° 

7820 

South  20°      

77.6° 

66.0° 

73  Q° 

South  30°      
South  40° 

70.0° 
«  i° 

57.1° 

460° 

64.0° 
S40° 

South  50°      
South  60°      

47-4° 
+  34-9° 

+36.8° 

0^-w 

+  41-5° 

Up  to  about  45°  latitude  the  northern  hemisphere  is 
warmer  than  the  southern,  but  beyond  that  latitude  the 
southern  hemisphere  is  the  warmer.  The  warmest  par- 
allel for  the  year,  or  the  thermal  equator,  is  found  to  be 
about  10°  north  of  the  equator. 

The  rate  of  poleward  decrease  in  the  temperature  is  irregular,  since 
the  relative  amounts  of  land  and  water  vary.  The  differences  between 
the  midwinter  and  midsummer  temperatures  for  the  two  hemispheres  are 
found  to  be  very  unequal ;  but  the  average  for  the  January  and  July 
temperatures  gives  nearly  the  annual  temperature  in  the  respective 
hemispheres. 


(5?; 


58  ELEMENTARY   METEOROLOGY. 

The  average  surface  air  temperature  of  both  the  northern 
and  southern  hemispheres  is  about  59°  F.  For  the  extreme 
months  January  and  July  the  average  temperatures  have 
been  computed  as  follows  :  — 


TANTAKY. 

JULY. 

F. 
46.4° 

F. 

72  C° 

Southern  hemisphere  

63  *0 

HA  1° 

Whole  earth 

«o° 

J^O 
6l  1° 

jj'^ 

UO'J 

It  has  also  been  found,  that,  on  account  of  the  greater 
amount  of  land  in  the  eastern  hemisphere  (counting  this 
from  80°  west  longitude  to  100°  east  longitude  from  Green- 
wich), the  eastern  hemisphere  is  about  2°  F.  warmer  than 
the  western. 

Abnormal  Average  Temperatures. — The  irregular  distri- 
bution of  temperatures  over  the  earth  is  best  shown  by 
considering  the  difference  between  the  average  tempera- 
ture of  each  parallel  and  the  actual  temperature  at  places 
along  the  same  parallel.  These  differences  are  called  the 
abnormal  temperatures  of  those  places.  The  abnormal 
temperatures  are  charted,  and  the  lines  of  equal  magni- 
tudes drawn  on  the  charts  are  called  is-abnormals. 

For  the  whole  year  and  for  the  winter  the  temperatures 
are  abnormally  highest  over  the  eastern  parts  of  the  oceans, 
and  abnormally  lowest  over  the  centers  of  the  continents  in 
northern  latitudes. 

For  the  summer  the  temperatures  are  abnormally  high- 
est over  the  centers  of  the  continents,  and  abnormally  lowest 
over  the  eastern  parts  of  the  oceans  in  northern  latitudes. 
Such  charts  are  shown,  for  January  in  Fig.  12,  and  for  July 
in  Fig.  13. 


(59) 


60  ELEMENTARY   METEOROLOGY. 

Abnormal  Temperatures  for  the  Year.  —  The  regions  where  the  local 
temperatures  depart  most  below  the  normal  are  in  north-central  North 
America  (-I2°F.)  and  northeastern  Asia  (-i8°F.).  The  regions 
where  the  local  temperatures  depart  most  above  the  normal  are  off  the 
coast  of  Norway  (  +  20°  F.),  around  the  eastern  end  of  the  Mediter- 
ranean Sea  (  +  10°  F.),  and  on  the  southern  Alaskan  coast  (  +  83  F.). 

Abnormal  Temperatures  in  January.  —  In  the  chart  (Fig.  12),  the 
minimum  negative  abnormal  temperatures  are  —30°  F.  in  northeastern 
Asia,  between  —20°  and  —  30^  F.  in  north-central  North  America,  and 
— 10°  F.  to  the  west  of  the  middle  latitudes  of  South  America  and  the 
southern  latitudes  of  Africa.  The  maximum  positive  abnormal  tem- 
peratures are  +40°  F.  off  the  western  coast  of  Norway,  +20°  F.  off 
the  southern  coast  of  Alaska,  and  + 10°  F.  in  central  Australia, 
southern  South  America,  and  southern  Africa. 

Abnormal  Temperatures  in  July.  — In  the  chart  (Fig.  13),  the  mini- 
mum negative  abnormal  temperatures  are  — 10°  F.  on  the  northeastern 
part  of  the  Pacific  Ocean,  on  the  North  Atlantic  between  southern 
Greenland  and  the  North  American  Continent,  and  to  the  west  of 
South  America  and  Africa  a  few  degrees  south  of  the  equator.  The 
maximum  positive  abnormal  temperatures  are  -f  20°  F.  in  the  north- 
western United  States  (inland),  + 10°  F.  in  central  Asia,  and  + 10°  F. 
over  the  desert  of  northern  Africa.  In  the  southern  hemisphere  there 
is  a  slight  negative  abnormal  temperature  in  central  Australia.  Over 
the  greater  parts  of  South  America  and  Africa  there  is  a  slight  positive 
abnormal  temperature  both  in  winter  and  summer,  while  over  the  ocean 
to  the  west  of  these  the  air  is  abnormally  cool. 

Annual  Range  of  Average  Monthly  Temperatures.  — The 
difference  in  the  average  temperatures  for  the  coldest  and 
warmest  month  is  called  the  range  in  the  monthly  tem- 
peratures for  the  year.  It  increases  with  the  latitude  and 
towards  the  interior  of  continents,  but  decreases  with  the 
altitude. 

These  temperature  differences  for  many  localities  have 
been  deduced  and  entered  on  a  chart,  and  the  lines  of 
equal  range  of  temperature  have  been  drawn  by  connect- 
ing adjacent  places  having  the  equal  differences  of  tem- 


TEMPERATURE.  63 

perature.  Such  a  chart  is  given  in  Fig.  14;  and  it  shows 
that  along  the  equator  the  annual  range  of  temperature 
is  5°  F. ;  and  there  is  an  increase  with  the  distance  from 
the  equator,  slowly  over  the  oceans,  but  rapidly  over  the 
interiors  of  the  continents. 

In  the  southern  hemisphere  the  temperature  range  reaches  30°  F. 
in  Africa,  South  America,  and  Australia.  In  the  northern  hemisphere 
the  temperature  range  increases,  at  first  slowly,  and  then  at  the  extreme 
north  rapidly,  to  about  40°  F.  in  the  Pacific  and  Atlantic  oceans ;  but 
the  increase,  is  to  80°  F.  in  north-central  North  America,  and  to  120°  F, 
in  northeastern  Asia. 

The  5°  F.  line,  extending  nearly  across  the  South  Pacific  Ocean, 
shows  a  remarkable  instance  of  uniformity  in  the  temperature  range 
over  a  water  surface ;  and  the  5°  F.  line  in  Africa,  and  the  40°  F.  and 
the  70°  F.  line  in  southern  and  central  Asia,  show  for  the  interior  of 
continents  a  like  parallelism  with  the  circles  of  latitude. 

Charts  of  Average  Highest  and  Lowest  Temperatures  for 
the  Year.  —  The  accompanying  charts  (Figs.  15  and  16) 
show  the  average  maximum  and  average  minimum  air 
temperatures  for  the  year  over  the  whole  earth.  The  lines 
of  equal  maximum  and  minimum  temperatures  are  drawn 
for  each  9°  F.  It  must  be  understood  that  the  charts 
represent  the  averages,  for  a  number  of  years,  of  the 
extreme  highest  and  lowest  temperatures  occurring  in 
each  year. 

The  average  highest  temperature  for  the  year  decreases 
with  increase  of  latitude  and  altitude,  but  increases  towards 
the  interior  of  continents.  The  average  lowest  temperature 
for  the  year  becomes  lower  with  increase  of  latitude  and 

towards  the  interior  of  continents. 

% 
Chart  of  Average  Extreme  Maximum  Temperatures  for  the  Year  (Fig. 

15).  —  This  chart  shows  that  the  maximum  temperatures  have  a  quite 
regular  distribution  on  the  oceans,  where,  for  wide  zones  extending  both 
sides  of  the  equator,  there  is  a  maximum  of  about  86°  F. ;  and  in  no 


TEMPERATURE.  65 

case  does  it  reach  95°  F.  From  these  zones  poleward  there  is  a  rela- 
tively rapid  decrease  with  the  latitude ;  and  the  temperature  of  68°  F. 
is  reached  near  60°  latitude  in  the  northern,  and  50°  latitude  in  the 
southern  hemisphere. 

The  continental  distribution  is  quite  different,  however,  for  there  an 
increase  of  the  maximum  temperatures  takes  place  with  the  progress 
inland.  In  the  interior  of  northern  Africa,  Persia,  northern  India,  Aus- 
tralia, and  southern  North  America,  temperatures  of  113°  F.  are  to  be 
found ;  and  in  the  southwestern  United  States,  and  perhaps  in  the 
Sahara  desert,  they  even  reach  122°  F. 

At  high  altitudes  the  extreme  ranges  of  the  maxima  are  less  than  on 
the  lowlands,  and  are  thus  more  like  those  for  marine  localities. 

Chart  of  Average  Extreme  Minimum  Temperatures  for  the  Year.  — 
This  chart  (Fig.  16)  shows  very  strongly  marked  regional  character- 
istics. On  the  Pacific,  Atlantic,  and  Indian  oceans,  in  the  equatorial 
region,  there  are  extensive  zones  stretching  from  west  to  east,  in  which 
the  minimum  temperatures  do  not  go  below  68'  F.  To  the  north  of 
these  there  is  a  rapid  decrease  in  the  minimum  temperatures,  but  to  the 
south  it  is  not  so  rapid. 

There  is  rapid  decrease  towards  the  interior  of  the  continents,  and 
especially  where  there  exist  mountain  ranges  to  cut  off  the  access  of 
the  sea  air  to  the  interior.  In  the  northern  hemisphere  there  are  three 
centers  of  extreme  minimum  temperatures,  —  one  in  the  eastern  part  of 
Siberia,  another  in  the  northern  part  of  North  America,  and  a  third  in 
the  interior  of  Greenland. 

The  32°  F.  line  is  very  interesting  as  showing  the  limits  of  the  region 
in  which  the  freezing  point  of  water  is  reached.  In  the  northern  hemi- 
sphere, commencing  at  the  Yellow  Sea,  we  can  follow  its  easterly  course 
with  a  northern  bend  on  the  Pacific  Ocean,  cutting  North  America  at 
about  latitude  30°.  On  the  Atlantic  Ocean,  following  the  Gulf  Stream 
for  a  distance  in  a  northeasterly  direction,  it  reaches  almost  to  Ireland, 
when  it  takes  a  southeasterly  trend,  and  passes  along  the  coast  of  the 
Spanish  peninsula.  Continuing  through  the  Mediterranean  Sea,  it 
passes  through  southern  Asia  to  the  place  from  which  we  started  in 
tracing  it.  In  the  southern  hemisphere  it  makes  almost  a  loop  within 
the  continent  of  Australia,  and  passes  from  its  southeastern  corner  in 
an  easterly  direction  through  northern  New  Zealand,  and  thence  onward 
until  the  southern  end  of  South  America  is  almost  reached ;  but,  stop- 
ping short  of  it,  the  line  suddenly  makes  a  northward  curve  up  along 


TEMPERATURE.  67 

the  coast  to  about  latitude  20°  south,  where  it  crosses  the  continent, 
and  then  runs  southward  again  along  the  eastern  coast  to  latitude  45° 
south,  and  thence  takes  an  easterly  course  to  the  southern  end  of 
Australia. 

Variations  in  altitude  above  sea  level  have  even  a  less 
influence  on  the  minimum  temperatures  than  on  the  maxi- 
mum temperatures,  as  frequently  in  the  severest  cold 
weather  there  is  an  inversion  of  the  temperatures  with  the 
altitude. 

Average  Annual  Oscillation  of  Temperature.  —  The  aver- 
age extreme  oscillation  or  amplitude  of  the  temperature 
during  the  year,  increases  with  the  latitude  and  towards 
the  interior  of  continents,  but  decreases  with  the  altitude. 

The  lines  of  equal  amounts  of  the  extreme  temperature 
oscillation  during  the  year  are  drawn  on  the  chart  (Fig.  17) 
at  intervals  of  9°  F. 

,  These  curves  bring  out  most  clearly  the  difference  between  the  con- 
tinental and  marine  climates.  On  the  ocean  near  the  equator  the 
absolute  temperature  amplitude  becomes  less  than  18°  F.  It  increases, 
however,  towards  the  poles  and  towards  the  continents,  and  especially 
towards  the  interior  of  the  continents.  In  eastern  Asia  an  average 
amplitude  of  171°  F.  (in  single  cases  over  180°  F.),  and  in  North 
America  153°  F.,  is  reached  ;  but  at  the  interior  of  Australia  the  ampli- 
tude is  only  90°  F.,  and  in  South  America  only  81°  F.  A  further 
description  of  these  charts  cannot  be  given  here,  but  they  will  well 
repay  a  careful  study.  In  many  respects  they  are  of  more  interest  than 
those  of  the  average  monthly  or  annual  temperatures. 

The  average  time  of  the  lowest  temperature  for  the  year  occurs  ear- 
lier for  a  continental  climate  than  for  a  marine  climate.  The  warmest 
and  coldest  days  of  the  year  do  not  occur  especially  frequently  on  the 
average  dates  of  extreme  temperature,  or  on  any  other  particular  dates- 

The  Snow  Line  as   Dependent  on  Temperature.  —  The 

snow  line,  or  line  of  eternal  snow,  is  that  line  along  which, 
in  the  course  of  a  year,  just  as  much  snow  falls  as  can  be 


TEMPERATURE.  69 

melted  by  the  sun  in  the  same  time.  Below  this  the  snow 
would  disappear ;  and  above  it,  it  would  remain  perma- 
nently present.  This  snow  line  is  not  fixed,  but  varies  in 
altitude  in  different  regions,  and  from  year  to  year  in  the 
same  region.  The  region  of  the  snow  line,  then,  is  an  alti- 
tudinal  zone  within  which  this  line  oscillates  up  and  down 
with  the  varying  temperature  of  the  seasons. 

The  limits  of  this  zone,  and  even  the  average  height  of  this  line, 
may  be  determined  not  only  by  means  of  many  years'  observations  of 
the  snow  phenomenon  direct,  but  also  by  means  of  the  study  of  per- 
manent glaciers.  These  last  must  extend  up  into  the  regions  of  eternal 
snow,  and  the  upper  limit  of  the  snow  line  is  lower  than  the  peak  sur- 
mounting the  glacier.  By  an  examination  of  the  surrounding  lower 
peaks,  which  are  favorably  situated  for  glacier  formations,  but  do  not 
have  them,  a  lower  limit  for  the  glacier  can  be  established ;  for  the 
snow  limit  must  be  above  these  glacierless  peaks,  and  below  the  peaks 
which  have  glaciers. 

The  temperature  at  the  limit  at  which  the  snow  disap- 
pears depends  a  good  deal  on  the  amount  of  snow  which 
has  accumulated.  The  mean  annual  temperature  at  the 
lower  limit  of  the  snow  line  varies  from  37°  to  3°  F.  for 
different  regions  for  which  we  have  observations ;  and  it 
may  be  said,  that,  the  greater  the  differences  between 
the  winter  and  summer  temperatures,  the  less  will  be  the 
average  annual  temperature  at  which  snow  is  always 
found.  Observations  covering  a  number  of  years  are 
necessary  for  determining  the  average  position  of  the 
snow  limit,  which  is  more  constant  in  •  lower  than  in 
higher  latitudes. 

The  average  annual  temperature  at  which  the  snow  line  is  found 
decreases  towards  the  pole,  as  is  seen  from  the  table  given  on  the 
next  page.  The  snow  line  is  found  nearer  the  sea  level  as  the  cold 
polar  regions  are  approached. 


ELEMENTARY   METEOROLOGY. 


There  has  been  a  great  deal  of  data,  of  more  or  less  accuracy,  accu- 
mulated concerning  the  snow  limit  in  various  parts  of  the  world,  but 
the  following  table  will  suffice  for  reproduction  here  :  — 


MOUNTAINS. 

GEOGRAPHICAL 
LATITUDE. 

ALTITUDE  OF 
SNOW  LIMIT  ABOVE 
SEA  LEVEL. 

AVERAGE 
ANNUAL  TEM- 
PERATURE. 

Feet. 

F.° 
about 

Andes  (near  Quito)      .     . 

Equator. 

i5»5°o 

34 

East  African  Mts.    .     .     . 

Near  Equator. 

I5.500 

Himalaya  (south  side)     . 

27°-34°  N. 

l6,OOO 

3i 

Himalaya  (north  side)     . 

27°-34°  N. 

18,500 

27 

Middle  and  West  Alps      . 

46°  N. 

9,000 

27 

Tyrolean  Alps  (central)     . 

47°  N. 

9,000  + 

25 

Nova  Zembla       .... 

73°  N. 

2,000 

12 

Spitzbergen          .... 

77°  N. 

I,  COO 

14 

/  / 

*o 

T" 

Temperature  of  the  Ground.  —  The  heat  of  the  ground 
at  or  near  the  earth's  surface  is  received  from  two  sources, 
—  the  interior  of  the  earth,  and  the  sun.  The  heat  of  the 
earth  increases  towards  the  interior,  and  there  is  a  flow  of 
heat  by  conduction  from  the  interior  outward  to  supply 
the  heat  lost  by  radiation  from  the  surface  of  the  earth. 
The  heat  from  the  sun  raises  the  temperature  of  the 
earth's  surface  higher  than  that  due  to  the  heat  received 
only  from  within.  According  to  the  law  for  the  conduc- 
tion of  heat,  there  must,  then,  be  a  transference  of  heat 
from  the  earth's  surface  towards  the  interior  of  the  earth, 
and  there  is  a  decrease  in  the  temperature  of  the  suc- 
cessive layers  of  earth  as  far  as  the  heat  from  the  sur- 
face penetrates ;  and  from  this  point  inwards  towards  the 
center  of  the  earth  there  is  an  increase  of  temperature. 

The  diurnal  changes  of  ground  temperature^  following 
those   of  the  air  above,  are  greatest  at  the  equator,  and 


TEMPERATURE.  71 

decrease  towards  the  pole,  and  are  noticeable  to  a  depth 
of  about  40  inches  at  the  equator.  With  descent  below 
the  surface,  the  times  of  maximum  and  minimum  temper- 
ature are  retarded. 

The  annual  changes  of  ground  temperature  follow  after 
those  of  the  air  above  to  a  depth  of  about  80  feet;  but 
the  times  of  maximum  and  minimum  temperature  are 
retarded  as  much  as  several  months  at  such  depths. 

The  average  ground  temperature  at  a  depth  of  about  3  feet  below 
the  surface  is  about  2°  F.  higher  than  the  air  temperature  above.  The 
normal  increase  in  the  earth  temperature  at  greater  depths  is  about 
i°  F.  for  each  60  feet  descent ;  but  very  different  rates  of  increase 
have  been  observed  in  various  parts  of  the  world. 

The  Region  of  Frozen  Earth,  where,  at  some  depth  below 
the  earth's  surface,  there  is  a  continuous  temperature  of 
32°  F.  or  a  lower  temperature,  occurs  in  the  polar  regions 
and  some  very  elevated  mountains  on  other  parts  of  the 
earth.  The  southern  limit  in  the  northern  hemisphere 
at  which  the  ground  is  perpetually  frozen,  is  along  the 
annual  isotherm  of  about  28.5°  F.  for  air  temperatures 
at  sea  level. 

Temperature  of  the  Ocean.  —  Observations  of  the  sur- 
face temperatures  of  the  ocean  water  exhibit  a  daily 
change  somewhat  similar  to  that  of  the  air  above  it;  but 
the  amplitude  or  range  amounts  to  only  a  fraction  of  a 
degree  during  the  24  hours.  The  maximum  temperature 
occurs  shortly  after  midday,  and  the  minimum  just  after 
sunrise. 

The  annual  change  of  the  ocean  surface  temperatures 
follows  that  of  the  air;  but  the  amplitude  is  much  less, 
and  increases  with  the  latitude  up  to  high  middle  latitudes, 
where  it  decreases  again.  The  times  of  maximum  and 


72  ELEMENTARY   METEOROLOGY. 

minimum  temperatures  are  retarded  about  a  month  later 
than  those  for  the  air  temperatures,  and  occur  in  August 
and  February  respectively,  in  the  northern  hemisphere. 

In  the  warmest  parts  of  the  tropical  zone  the  surface  temperature 
changes  very  little  during  the  year,  and  is  about  82°-84°  F.  In  middle 
latitudes  it  varies  from  about  50°  F.  in  winter  to  68°  F.  in  summer.  In 
the  polar  zone  the  temperature  approaches  or  goes  below  the  freezing 
point  of  fresh  water,  and  varies  but  little  with  the  season. 

Temperatures  of  Small  Isolated  Bodies  of  Water.  —  The 

distribution  of  temperature  in  the  lakes  is  regulated  by  the 
currents  which  arise  from  the  alternate  heating  and  cool- 
ing of  the  surface  water.  The  warm  surface  water  in 
summer  imparts  little  of  its  heat  to  the  water  below,  which 
consequently  does  not  get  much  above  39°  F.  for  deep 
lakes.  There  is,  however,  a  condition  of  stable  equilib- 
rium, because  the  temperature  decreases  with  the  depth, 
and  the  coldest  and  heaviest  water  is  at  the  bottom.  In 
the  fall  and  early  winter  the  surface  cools  to  about  39°  F., 
at  which  temperature  water  is  heaviest ;  and  then,  when 
the  whole  body  of  water  reaches  this  temperature,  the 
water  is  in  indifferent  equilibrium,  and  there  are  no  ver- 
tical currents.  When  the  surface  water  becomes  still 
colder,  it  becomes  lighter  than  the  water  below,  and 
does  not  sink. 


CHAPTER   III. 
AIR  PRESSURE. 

Nature  of  Air  Pressure.  —  The  atmospheric  air,  obeying 
the  law  of  gases,  exerts  a  pressure  in  all  directions,  and 
the  amount  of  this  pressure  varies  according  to  the  den- 
sity of  the  air.  The  air  at  the  sea  level,  weighted  down 
by  the  air  above  it,  exerts  a  pressure  of  nearly  1 5  pounds 
per  square  inch  of  surface  against  which  it  presses. 

The  air  pressure  decreases  with  increase  of  altitude, 
because  there  is  less  air  above,  the  higher  the  ascent.  We 
do  not  know  just  how  far  from  the  earth's  surface  the  air 
pressure  is  a  measurable  quantity,  but  at  an  altitude  of 
a  few  miles  it  becomes  very  small.  Where  the  free  air 
has  access  below  the  earth's  surface,  there  is  a  continued 
increase  of  air  pressure  with  the  descent.  Also  at  various 
points  on  the  earth's  surface  the  air  pressures  are  not 
quite  the  same ;  and  for  any  one  point  the  pressure 
undergoes  variations  of  the  nature  of  oscillations  occurring 
in  shorter  or  longer  periods  of  time. 

Distribution  of  Air  Pressure.  —  The  pressure  of  the  air — 
called  the  barometric  pressure,  from  the  instrument  used 
for  measuring  it  —  would  be  symmetrical,  and  would  be 
practically  the  same  at  all  places  having  a  common  alti- 
tude above  the  level  of  the  sea,  if  it  were  not  for  the  dis- 
turbing influence  of  the  solar  heat  and  of  the  movements 

73 


74 


ELEMENTARY   METEOROLOGY. 


of  the  air  caused  by  its  unequal  heating  at  different  places 
on  the  earth. 

If,  under  the  condition  of  constant  temperature,  it  is  necessary  to 
ascend  a  certain  distance  above  the  earth's  surface  in  order  to  leave 
say  TXO  °f  tne  atmosphere  below  and  have  the  other  T9<y  above,  then  it 
is  necessary  to  ascend  the  same  distance  in  order  to  leave  below  ^  of 


FIG.  18.  —  DECREASE  OF  DENSITY  AND  AMOUNT  OF  AIR  WITH  INCREASE  OF  ALTITUDE. 

the  remainder,  or  T^  of  -*„  of  the  whole  (in  addition  to  what  was  left 
below  the  first  time)  ;  and  so  on  up  to  any  limit. 

In  the  actual  case  of  the  atmosphere  there  is  a  decrease  of  tem- 
perature with  the  increase  of  altitude,  and  the  air  consequently  becomes 
rarer  at  a  progressively  more  rapid  rate  as  the  ascent  is  made ;  so  that 
it  is  not  necessary  at  higher  elevations  to  go  through  so  great  a 
distance  to  get  above  a  given  proportion  of  the  whole  air  as  at  lower 
elevations,  where  the  air  is  warmer. 


AIR   PRESSURE.  7$ 

The  accompanying  diagram  (Fig.  18)  shows  roughly 
the  relative  density  of  the  air  at  various  altitudes  above 
sea  level  up  to  about  the  probable  limit  where  the  pres- 
sure of  the  air  ceases  to  be  a  barometrically  measurable 
quantity. 

On  the  right  of  the  diagram  is  shown  the  relation  of  the  highest 
mountains  to  these  altitudes.  The  column  on  the  left,  showing  the 
pressure  of  the  air  at  the  various  altitudes,  will  be  better  understood 
after  reading  the  descriptions  of  barometers,  the  instruments  by  which 
such  measurements  are  made.  It  is  seen  that  the  tops  of  the  highest 
mountains  have  nearly  three  fourths  of  the  total  amount  (by  weight) 
of  the  air  below  them ;  and  at  the  highest  altitudes  permanently 
inhabited  by  human  beings  about  half  the  air  is  left  below. 

The  accompanying  table  shows  the  relation  of  air  pressure  and 
altitude  for  every  change  of  two  inches  in  pressure  from  30  to  16 
inches.  The  altitudes  are  given  in  round  numbers  to  the  nearest 
100  feet. 

ALTITUDE 
ABOVE  SEA  LEVEL.  BAROMETRIC  PRESSURE. 

FEET.  INCHES. 

o 30 

1,800 28 

3,800 26 

5>9°° 24 

8,200 22 

10,000 20 

13,200 18 

16,000 .     16 

The  Barometer.  —  Galileo  discovered,  over  two  hundred 
and  fifty  years  ago,  that  the  air  exerts  its  elastic  pressure 
in  all  directions.  Shortly  afterwards  (in  1643)  Torricelli 
gave  us  an  easy  method  of  measuring  this  air  pressure, 
or  barometric  pressure  as  we  shall  usually  call  it,  when 
he  invented  the  barometer. 

The  simplest  form  of  barometer  is  obtained  by  filling 

WALDO    METEOR.  —  5 


ELEMENTARY   METEOROLOGY. 


with  mercury  a  glass  tube  of  a  length  of,  say,  three  feet- 
and  one  third  of  an  inch  in  diameter,  and  closed  at  one 
end.  Hold  the  closed  end  uppermost  and  insert  the 
mouth  of  the  tube  in  a  cup  also  containing  mercury  (Fig. 
19).  On  making  the  tube  vertical,  some  of  the  mercury 
will  flow  downward  out  of  the  tube  into 
the  cup,  until  the  weight  of  the  mercury 
in  the  tube  is  counterpoised  by  the  hydro- 
static pressure  of  the  air  on  the  surface 
of  mercury  in  the  open  cup. 

The  height  of  the  surface  of  the 
mercury  in  the  tube  above  the  sur- 
face of  the  mercury  in  the  cup  is  meas- 
ured by  means  of  a  scale  of  length ; 
and  the  amount,  expressed  in  inches, 
is  called  the  barometer  keig/tt,  or  the 
barometric  pressure •,  or  merely  the  barom- 
eter reading. 

In  making  accurate  determinations  of  the  baro- 
metric pressure,  several  sources  of  error  must  be 
allowed  for.  If  proper  care  is  taken  in  the  manip- 
ulation of  the  tube,  the  space  above  the  mercury 
in  the  barometer  tube  will  be  practically  a  vacuum, 
and  consequently  there  will  be  no  pressure  on  the 
top  surface  of  the  mercury  in  the  tube.  Fig.  19 
shows  on  the  left  a  barometer  tube  situated  as 
just  described.  The  figure  on  the  right  shows 

the  position  the  mercury  would  take  if  the  tube 
FIG.  19.  —  SIMPLE  FORMS  ; 

OF  BAROMETERS.  had  a  U  bend  at  the  bottom,  in  which  case  the 
mercury  cup  would  be  unnecessary.  The  distance 
AB  is  about  30  inches  under  ordinary  air  pressures  at  sea  level. 

Fig.  20  shows  an  actual  barometer  where  the  glass  tube  is  incased  in 
a  metal  tube  which  serves  not  only  to  protect  it,  but  also  for  a  measuring 
sca!e.  The  cup  or  cistern  of  mercury  is  at  A.  By  means  of  the  screw 
C  the  mercury  is  forced  up  until  the  mercury  surface  in  A  is  brought 


AIR  PRESSURE. 


77 


FIG    20  -  COMPLETE     fixed    altitude. 
MERCURIAL  BAROM- 


to  the  little  pointer  shown  at  B,  which  is  the  zero 
or  beginning  of  the  barometer  scale.  The  height 
of  the  mercury  in  the  tube  is  then  read  off  from 
the  scale  at  C.  D  is  an  attached  thermometer. 

Fig.  21  shows  an  aneroid  barometer.  This  is 
an  instrument  shaped  something  like  a  small  clock, 
and  the  barometric  pressure  is  read  by  means  of 
a  hand  or  indicator,  on  a  scale  of  inches  placed 
on  the  face  like  the  minute  divisions  of  a  clock. 
A  little  flat,  circular,  metal  box  within  the  case  is 
partly  exhausted  of  air, 
and  tightly  sealed  ;  and 
when  the  outer  air 
pressure  increases,  it 
forces  in  the  side  of 
the  box,  and  this  mo- 
tion is  communicated 
to  the  hand,  making 
it  move  around  toward 
the  right.  When  the 
pressure  is  lessened, 
the  hand  moves  to  the 
left.  Another  hand,  FIG.  21. -ANEROID  BAROMETER. 
which  is  usually  placed  on  the  case,  remains  station- 
ary unless  moved  by  some  person. 

The  Reduction  of  Barometric  Pressure  to 
Sea  Level.  —  This  is  one  of  the  most  un- 
satisfactory problems  connected  with  prac- 
tical meteorology.  The  observations  of 
air  pressure  are  usually  made  at  places 
having  various  (known)  altitudes ;  and, 
since  the  air  pressure  decreases  with  alti- 
tude, then,  in  order  to  compare  the  results 
from  these  places,  the  pressures  must  be 
reduced  to  some  common  plane  having  a 
For  convenience  the  sea 


78  ELEMENTARY   METEOROLOGY. 

level  has  been  chosen  as  a  level  for  reduction,  but  the 
observations  could  be  reduced  to  any  other  desired  level, 
and  sometimes  other  levels  are  chosen. 

Closely  connected  with  this  matter  is  the  determination 
of  the  difference  in  the  altitude  of  two  places  by  means  of 
the  observations  of  the  air  pressure  at  both  places.  We 
can  suppose  observations  to  be  made  on  various  parts  of 
the  mountains  shown  in  Fig.  18.  If  the  barometric  pres- 
sure is  observed  in  a  balloon  at  B,  and  we  wish  to  reduce 
this  to  the  corresponding  pressure  at  sea  level,  we  should 
add  it  to  the  weight  of  the  air  column  BC  (expressed  in 
barometric  pressure);  and  similarly  for  reducing  baro- 
metric pressure  observed  at  A  to  sea  level  at  E,  we  must 
add  to  it  the  weight  of  an  assumed  column  of  air,  AE. 

The  problem  of  the  reduction  of  the  barometric  pressure  to  sea  level 
is  too  complicated  to  explain  in  full  here,  but  it  amounts  to  this,  —  that 
we  have  the  air  pressure  given  for  a  certain  altitude,  and  it  is  required  to 
find  the  weight  of  a  column  of  air  which  would  extend  from  this  alti- 
tude down  to  sea  level.  In  the  case  of  an  observation  in  a  balloon 
over  the  ocean,  there  would  be  a  real  column  of  air  to  sea  level ;  but  in 
the  case  of  an  observation  on  a  mid-continental  mountain,  the  column 
of  air  would  be  purely  fictitious.  In  order  to  determine  the  weight 
of  the  intervening  column  of  air,  it  is  necessary  to  know  its  average 
temperature.  This  might  be  determined  in  the  case  of  the  balloon  ;  but 
for  the  mountain  the  local  temperature  of  the  region  must  be  taken  as 
the  upper  temperature,  and  from  this  the  temperature  at  the  assumed 
sea  level  below  must  be  calculated,  in  order  to  obtain  the  average  tem- 
perature of  the  fictitious  air  column,  so  that  its  weight  can  be  com- 
puted. The  laws  of  the  decrease  of  air  temperature  with  altitude  are 
so  different  for  different  regions,  that  there  is  room  for  considerable 
error  in  obtaining  the  average  temperature  of  the  air  column  by  this 
means ;  and  in  proportion  as  this  is  in  error,  just  so  much  is  the  com- 
puted weight  of  the  air  column  in  error.  When  the  weight  of  the  air 
column  is  determined  on  the  barometric  scale,  this  amount  is  added 
to  the  corrected  observed  barometric  pressure,  and  the  sum  is  the 
barometric  pressure  reduced  to  sea  level.  Tables  for  facilitating  the 


AIR   PRESSURE.  79 

reduction  of  barometric  observations  to  sea  level  are  numerous.  Those 
in  the  "  Smithsonian  Meteorological  Tables  "  and  Hazen's  "  Meteoro- 
logical Tables  "  are  very  complete. 

The  Determination  of  the  Difference  in  Altitude  of  two 

neighboring  places  by  means  of  simultaneous  barometer 
observations  at  the  two  points,  is  the  reverse  of  this  reduc- 
tion from  one  known  level  to  another  known  level.  In 
this  case  the  barometric  pressure  and  air  temperature  are 
given  for  the  two  points ;  and  it  is  required  to  determine 
the  vertical  length  of  a  column  of  air  which  has  the  pres- 
sure and  temperature  of  the  higher  place  at  its  top,  and 
those  of  the  lower  place  at  its  base.  If  we  wish  to  deter- 
mine the  difference  in  altitude  between  A  and  D  (Fig.  18), 
we  should  observe  simultaneously  the  barometric  pressure 
at  D  and  -at  A  (which  would  be  the  same  as  at  F\  and 
the  difference  in  the  pressures  will  be  the  weight  (in  pres- 
sure) of  the  column  of  air  FD. 

We  can  determine  the  vertical  length  of  the  air  column  for  each 
inch  of  difference  in  these  barometric  pressures,  and,  by  combining 
their  whole  and  fractional  parts,  obtain  the  total  distance  correspond- 
ing to  the  whole  difference  of  pressures. 

Tables  are  also  used  to  facilitate  these  computations ;  and  since  the 
difference  in  the  altitudes  of  the  two  places  seldom  extends  from  high 
elevations  to  the  sea  level,  and  since  the  temperatures  of  the  lower 
station  are  known,  these  reductions  and  computations  are  usually 
capable  of  being  carried  out  with  greater  accuracy  than  the  reductions 
to  the  sea  level,  which  have  been  described  just  above. 

As  it  may  be  useful  to  make  a  rough  computation  of  the  reduction 
of  barometric  pressure  to  sea  level  when  the  altitude,  and  consequently 
the  vertical  length,  of  the  air  column,  is  known,  or  to  determine  the 
difference  in  altitude  when  the  approximate  height  of  the  column  of  air 
(equal  to  a  difference  in  barometer  readings)  is  given,  the  following 
table  is  offered  for  such  use.  It  gives  the  vertical  length,  in  feet,  of  a 
column  of  air  corresponding  to  o.i  of  an  inch  of  the  barometric  pres- 
sure at  various  air  pressures  and  temperatures. 


8o 


ELEMENTARY   METEOROLOGY. 


VERTICAL  LENGTH  OF  AN  AIR  COLUMN  CORRESPONDING  TO  o.i  OF  AN 
INCH  BAROMETRIC  PRESSURE. 

AVERAGE  TEMPERATURE  »IN  DEGREES  FAHRENHEIT. 


20° 

30° 

4<F 

50° 

60° 

70° 

803 

22         .  '    

Feet. 

116 

Feet. 
I  IQ 

Feet. 
122 

Feet. 
124. 

Feet. 
127 

Feet. 
I  "?O 

Feet. 
I  "*2 

->-?                                      .    ' 

III 

I  14. 

116 

I  IQ 

124. 

124 

126 

*A 

1  06 

IOQ 

III 

114. 

116 

121 

121 

2C 

IO2 

IOC 

IO7 

IOQ 

I  12 

114. 

116 

*J 

26 

Q8 

IW5 

IOI 

IO"? 

iwy 

IOC 

IO7 

I  IO 

1  12 

2?         
28         

yu 

94 

QI 

97 

Q-J 

1WJ 

99 

QC 

IVO 
IOI 
Q8 

103 
IOO 

1  06 
IO'' 

1  08 
I  O4. 

2Q 

88 

QO 

Q2 

Q4. 

q6 

Q8 

IOO 

^V 

•JO 

8c 

87 

8q 

QI 

yvj 
QI 

V" 
QC 

Q7 

"3 

uy 

V1 

yj 

y.> 

y/ 

Thus,  when  the  barometer  reads  28  inches  and  the  temperature  is 
50°  F.,  then  at  a  point  98  feet  lower  in  altitude  the  barometer  will 
read  o.i  of  an  inch  more  than  28  inches,  or  28.1  inches;  or  at  a  point 
98  feet  higher  it  will  read  27.9  inches. 

Results  of  Observations  of  Atmospheric  Pressure.  —  The 
observations  of  barometric  air  pressure  are  not  so  fre- 
quent as  those  of  air  temperature,  on  account  of  the  cost, 
complexity,  and  fragile  construction  of  barometers.  Still, 
enough  observations  have  been  made  to  allow  the  con- 
ditions and  changes  of  the  atmospheric  pressure  to  be 
noted  in  most  of  the  accessible  parts  of  the  surface  of  the 
globe.  The  observed  conditions  of  atmospheric  pressure 
have  been  studied  locally,  that  is,  for  individual  places ; 
and  also  in  their  geographical  distribution,  that  is,  in  their 
connection  with  those  of  other  places. 

Where  but  a  single  place  is  concerned,  it  is  usual  to  consider  the 
observed  pressures,  correcting  them  for  instrumental  errors  only  ;  but 
where  the  geographical  distribution  is  to  be  considered,  it  is  necessary 
to  reduce  the  observations  to  a  common  level. 


AIR   PRESSURE. 


81 


Observed  barometric  pressures  at  individual  places  show 
the  existence  of  both  diurnal  and  annual  periodic  changes, 
somewhat  similar  to  those  found  for  temperatures,  but  in  a 
less  degree.  These  periodic  changes  are,  however,  masked 
by  the  far  greater  accidental  changes  which  must  be  elimi- 
nated (by  taking  the  average  of  many  observations)  before 
the  periodic  changes  are  visible. 

The  Diurnal  Change  or  March  of  the  Air  Pressure  does 
not  culminate,  like  the  temperature,  in  a  single  maximum 
with  a  corresponding  minimum,  but  there  are  two  maxima 
and  two  minima.  In  general,  the  minima  occur  at  about 
4  h.  and  16  h.  (4  A.M.  and  4  P.M.),  and  the  maxima  at  about 
10  h.  and  22  h.  (10  A.M.  and  10  P.M.). 

The  amplitudes  of  oscillation  vary  in  different  parts  of 
the  world,  but  are  always  small,  amounting  to  only  about 
0.15  of  an  inch  in  the  regions  of  greatest,  and  to  o.oi  of 
an  inch  in  the  regions  of  least  oscillation.  The  amplitude 
is  in  general  greatest  near  the  equatorial  regions,  and 
diminishes  towards  the  poles. 

The  hourly  pressures  for  a  few  stations  are  given  in  the  following 
table :  — 

TABLE  SHOWING  DAILY  MARCH  OF  THE  BAROMETRIC  PRESSURE  ON  THE 
AVERAGE  FOR  THE  YEAR  (hourly  barometric  pressures  in  Inches  and  hun- 
dredths} . 


I" 

2h 

3h 

4h 

5h 

6h 

7" 

8* 

9h 

ioh 

Ilh 

ia»> 

i3h 

Key  West,  Fla.  . 

30.07 

.06 

•05 

•°S 

•05 

.06 

.07 

.08 

.09 

.10 

.09 

.08 

30.07 

St.  Paul,  Minn.  . 

30-13 

-13 

•!3 

•r3 

.14 

.14 

.14 

•15 

•15 

•15 

•15 

.14 

30.12 

Fort  Conger  .     . 

29.844 

.846 

.847 

.849 

.849 

.848 

.847 

.846 

.844 

.842 

•839 

.838 

29.839 

I4h 

I5h 

I6h 

I7h 

i8h 

I9h 

2Qh 

2lh 

22h 

23  h 

24*1 

Av. 

Key  West,  Fla.  . 

30.05 

.04 

•03 

•03 

.04 

•05 

.06 

.07 

.08 

.08 

30.08 

30.06 

St.  Paul,  Minn.  . 

30.11 

.11 

.11 

.11 

.11 

.11 

.12 

.12 

•13 

•13 

30.13 

30-13 

Fort  Conger  .     . 

29.841 

.844 

.846 

.846 

.846 

.845 

.845 

•845 

.844 

.842 

29.843 

29.844 

82 


ELEMENTARY  METEOROLOGY, 


tnchesh 


It  is  to  be  noticed  that  for  the  Arctic  regions  it  is  necessary  to  give  the 
pressures  in  thousandths  of  an  inch  in  order  to  show  the  diurnal  change. 

In  the  accompanying  figure  (Fig.  22)  the  daily  march  of  the  baro- 
metric pressure  is  given  for  Key  West,  St.  Paul,  Fort  Conger  in  the 
Arctic  region,  and  for  Calcutta,  India.  In  this  diagram  the  average 
daily  barometric  pressure  is  entered  as  the  horizontal  line  marked  o.oo 
inches ;  and  the  departure  or  the  amount  above  or  below  this  at  the 
different  hours  is  shown  on  the  different  lines,  and  the  rise  and  fall  by 
the  course  of.  the  curves. 

The  principal  maximum  barometric  pressure  occurs  be- 
fore noon,  and  the  principal  minimum  after  noon.  During 

the  winter  season  they  approach 
noontime,  but  in  summer  they 
recede  from  it.  The  amplitudes 
of  the  diurnal  oscillations  are 
greater  during  the  warmer  than 
during  the  cooler  part  of  the  day, 
and  for  the  dry  than  for  the  moist 
continental  localities.  They  di- 
minish from  the  equatorial  region 
of  greatest  solar  heat  towards  the 
poles.  The  cause  of  these  oscil- 

FIG.  22"! DIURNAL  CHANGES  OF  AIR  lations  is  not  at  present  perfectly 
PRESSURE.  understood  \  but  they  may  be  due 

to  waves  in  the  atmosphere  which  depend  in  some  way 
on  the  intensity  and  distribution  of  the  solar  heat. 

The  Annual  Change  or  March  of  the  Air  Pressure,  that  is, 
the  change  in  the  average  pressure  from  month  to  month, 
presents  a  variety  of  characteristics  for  different  regions 
of  the  earth.  On  the  continents  at  the  lower  altitudes 
there  is  a  maximum  air  pressure  in  winter,  and  a  minimum 
in  summer.  On  the  ocean,  on  the  contrary,  the  air  pres- 
sure is  highest  in  summer,  and  lowest  in  winter ;  and  this 
is  also  true  of  the  air  pressure  at  high  altitudes. 


0.05 

West 
vtta 
0.00 
nger 

005 

; 

\ 

/ 

1 

\ 

L 

\ 

'/ 

\ 

\ 

/C 

\v. 

5* 

4 

vv 

__ 

_ 

»J 

V 

>- 

V 

/ 

2  "" 

\ 

/' 

VI 

\ 

(/ 

s 

/ 

; 

i 

, 

\ 

0.10 

• 

2 

AIR  PRESSURE. 


This  matter  cannot  be  fully  understood  until  the  motions 
of  the  atmosphere  are  studied ;  but  it  may  be  said  in  gen- 
eral, that  over  the  continents  in  the  summer  the  air 
becomes  very  much  heated,  and  the  surfaces  of  equal  air 
pressure  more  elevated,  which  causes  the  air  to  flow  out- 
ward (above)  towards  the  oceans,  while  the  air  must  flow 
inward  (below)  from  the  oceans  towards  the  continents 
to  compensate  for  the  outflow  above.  Thus  the  warmer 
continental  air  causes  a  deficiency  in  the  amount  of  air, 
and  consequently  a  diminished  air 
pressure,  over  the  continent. 

The  greatest  and  most  regularly 
occurring  variations  in  the  mean  air 
pressure  take  place  on  the  conti- 
nents, and  the  least  on  the  oceans  and 
their  coasts.  There  are,  however, 
many  local  deviations  from  this  law. 


*J~F  M  AM  J  J  A 


fey  West 

St.Paul 

30.00 

Calcutta 

Atlantic 

Ocean 
Ft.Conger 


Jjj^ 

5 

*s 

<^- 

^ 

-^ 

s- 

H 

n* 

-j 

\s 

~ 

\ 

yf 

•*^ 

\  ' 

x 

/ 

\ 

7 

FIG.  23.  —  ANNUAL  CHANGES  OF 
AIR  PRESSURE. 


In  the  interior  of  Asia  the  amplitude  of  oscillations  amounts  to 
over  0.70  of  an  inch  in  a  year ;  while  over  the  Atlantic  Ocean  near 
the  equator  it  is  only  about  o.io  of  an  inch. 

The  following  table  shows  the  average  monthly  air  pressures  at  a 
few  points  on  the  earth's  surface  :  — 

ANNUAL  MARCH  OF  AIR  PRESSURE  {monthly  barometer  pressure  in 
inches  and  hundredths}. 


Jan. 

Feb. 

Mar. 

Apr. 

May 

June 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec.  Year 

Calcutta      .     .     . 

30  01 

29-95 

29.86  29.76 

29.70 

29-58 

29-57 

29.62 

29.71 

29.84 

29-95 

30.01 

29.80 

Key  West    .     .     . 

30.14 

30.12 

30.10  30.04 

30.01  30.04 

30.06 

30.01 

29.99 

29.99 

30.06 

30.12 

30-05 

St.  Paul  .... 

30.13130.10 

30.04  29.95  29.91 

2qqo 

29-93 

29-95 

29-97 

29.99 

30.04 

30.09 

29.99 

Fort  Conger     . 

29.80  29.67 

30.07  29.88  29.79 

29-83 

29-77 

29.9029.86 

29.92  29.88 

Atlantic  Ocean 

29-93  29-94 

29.9229.93 

29.94  29.98  30.04 

30.02 

30.01 

29.97  29.94 

29.92  29.96 

The  data  in  this  table  are  shown  graphically  in  Fig.  23. 

The  Irregular   or   Non-periodic   Oscillations   in   the   Air 
Pressure    are   those    changes    which    occur   more   or   less 


84 


ELEMENTARY    METEOROLOGY. 


gradually  from  time  to  time,  and  which  do  not  belong  to 
the  regular  diurnal  or  annual  periodic  changes.  They  are 
due  to  the  movement,  over  the  earth's  surface,  of  areas  of 
abnormally  high  or  low  barometric  pressure. 

The  monthly  barometer  oscillations  are  the  difference  between  the 
highest  and  lowest  barometer  reading  during  the  month.  The  follow- 
ing table  shows  the  average  barometric  oscillations  in  the  northern 
hemispheres,  over  the  sea  and  over  the  land,  for  the  winter  and  the 
summer  seasons. 

AVERAGE  MONTHLY  BAROMETRIC  OSCILLATIONS  (inches  and  hundredths) . 


IN  WINTER. 

IN  SUMMER. 

NORTH  LATITUDE. 

OVER  THB 

OVER  THE 

OVER  THE 

OVER  THE 

OCEAN. 

CONTINENTS. 

OCEAN. 

CONTINENTS. 

0° 

O.2O 

O.24 

0.20 

0.24 

10° 

O.24 

0.32 

O.2O 

0.24 

20° 

0.32 

o-43 

O.24 

0.32 

30° 

0.63 

0.51 

0-35 

0-43 

40° 

.14 

0.71 

0.63 

0.47 

50° 

•5° 

0.98 

0.98 

o-55 

60° 

•77 

1.22 

I.IO 

o-75 

70° 

•57 

I.I4 

0.98 

.  0.71 

80° 

•34 

0.71 

-~ 

The  place  of  greatest  oscillation  is  in  the  North  Atlantic  Ocean,  be- 
tween Newfoundland  and  the  British  Islands,  where  it  amounts  to 
nearly  2  inches  in  winter.  From  this  locality  there  is  a  decrease  in 
the  amplitude  of  oscillation,  quite  gradual  on  the  north,  east,  and  west 
sides,  but  very  rapid  towards  the  south  ;  and  in  the  neighborhood  of  the 
equator  it  becomes  almost  identical  with  the  periodic  daily  oscillations. 

These  barometer  oscillations  are  much  greater  over  the 
sea  than  over  the  land ;  and  greater  in  winter  than  in 
summer,  except  near  the  equator,  where  they  are  quite 
uniform,  as  may  be  seen  from  the  table  just  given. 


AIR  PRESSURE.  85 

Graphical  Representations  of  the  Distribution  of  Air  Pres- 
sures. —  An  isobaric  line,  or  an  isobar  as  it  is  usually 
called  by  meteorologists,  is  a  line  drawn  through  adjacent 
places  which  have  the  same  barometric  pressure.  In  order 
to  draw  isobaric  lines  on  a  chart,  the  barometric  pressures 
reduced  to  sea  level  are  written  down  on  the  map  at  the 
places  where  the  observations  are  made.  Then  the  iso- 
baric lines  are  drawn  on  the  map  usually  for  each  even 
tenth  of  an  inch  of  barometric  pressure. 

An  isobaric  surface  is  a  surface  in  the  air  all  points  of 
which  have  the  same  barometric  pressure. 

When  we  wish  to  show  the  position  or  distribution  of 
isobaric  lines  and  isobaric  surfaces  in  different  parts  of  the 
atmosphere  by  means  of  diagrams,  it  is  usual  to  take  some 
plane  or  level  surface,  such  as  the  sea  level  or  some  im- 
aginary surface  parallel  to  it,  and  designate  where  the 
isobaric  surfaces  intersect  this  plane,  and  the  positions 
which  they  assume  with  reference  to  it.  The  lines  where 
the  isobaric  surfaces  cut  this  plane  are  isobaric  lines,  or 
isobars. 

There  are  two  ways  in  which  the  level  of  isobaric  sur- 
faces is  disturbed :  - 

i.  Air  may  be  added  to  a  region  or  taken  from  it  by 
means  of  air  currents.  In  case  air  is  added,  the  pres- 
sure is  increased,  and  the  isobaric  surfaces  are  forced 
upward  from  the  earth's  surface,  and  lie  closer  together 
than  in  adjacent  regions  where  the  amount  of  air  is  dimin- 
ished or  unaltered.  In  case  air  is  abstracted,  the  pressure 
is  diminished,  and  the  isobaric  surfaces  are  forced  down- 
ward, and  lie  farther  apart  than  in  adjacent  regions.  Such 
changes  in  the  isobaric  surfaces  are  made  manifest  all  the 
way  down  to  the  earth's  surface,  and  at  the  ground  there 
is  an  increase  or  decrease  of  the  air  pressure. 


86  ELEMENTARY   METEOROLOGY. 

2.  The  quantity  of  air  over  any  given  region  may  re- 
main unchanged,  but  the  temperature  may  be  varied ;  in 
which  case  there  is  no  increase  or  diminution  of  the  air 
pressure  at  the  ground,  and  the  isobaric  surfaces  there 
remain  unchanged,  but  at  various  elevations  above  the 
ground  the  positions  of  the  isobaric  surfaces  will  depend 
on  the  distribution  of  the  temperature. 

This  last  condition  has  been  chosen  for  showing  in  more 
detail  the  nature  of  isobaric  surfaces  and  isobaric  lines. 

Let  us  suppose  that  we  are  up  in  the  air  at  some  distance  above  the 
ground,  and  that  we  take  the  plane  or  level  of  this  page  held  horizontally 
as  a  chosen  level  in  the  air,  having  a  barometer  reading  of  29  inches, 
for  example.  It  becomes  necessary,  then,  to  imagine  the  leaf  of  the  book 
to  be  so  perforated  that  the  air  can  pass  freely  through  it,  and  that  no  re- 
sistance is  offered  by  it  to  the  air  moving  either  upwards  or  downwards. 

Suppose  that  the  temperatures  are  uniform  on  this  level,  and  there 
are  also  common  temperatures  at  other  levels  above  and  below.  Then 
for  this  region,  inclosed  by  the  circle  in  Fig.  24  (i),  the  barometric 
pressures  are  29  inches.  Now,  let  the  temperature  of  the  air  become 
higher  in  the  center  at  A,  and  for  some  distance  above  and  below 
the  plane,  and  colder  on  the  outer  edge  at  Z?,  but  so  that  the  present 
condition  of  temperature  and  pressure  is  maintained  at  the  points  on 
a  circle,  C,  surrounding  A,  and  between  A  and  B,  Fig.  24  (2).  Then, 
since  warmer  air  is  expanded  and  colder  air  is  contracted,  the  warmer 
air  at  A  on  the  level  of  the  page  will  in  Fig.  24  (-2)  be  forced  upwards 
above  the  page  towards  the  reader's  eye,  while  the  colder  air  at  B  will 
be  contracted  below  or  beneath  the  page ;  but  at  C  it  will  remain  on 
the  level  of  the  page.  When  more  air  is  forced  above  the  plane  of  the 
page' at  the  center  A,  the  former  pressure  of  29  inches  on  the  page 
is  increased  by  the  amount  of  air  pushed  up :  while  on  the  outside, 
BB,  the  air  pressure  on  the  page  is  diminished  by  the  amount  of 
air  drawn  below  the  level  of  the  page.  But  around  the  circle  C,  where 
the  temperature  remains  the  same,  the  air  pressure  remains  29  inches. 

We  will  suppose  that  the  pressure  on  the  level  of  the  page  at  A  is 
increased  to  29.5  inches,  and  that  on  the  outskirts  BB  it  is  reduced  to 
28.5  inches,  then  we  shall  have  the  pressures  on  the  level  or  plane  of 
the  page  at  A,  £,  and  C,  as  shown  in  Fig.  24  (3). 


AIR   PRESSURE. 


The  pressure  on  the  level  of  the  page  will  then  be  greatest  at  the 
center  A,  and  will  gradually  decrease  to  the  outer  limits  B.  We  can 
then  also  draw  on  the  page  the  locations  where  an,y  of  the  air  pressures 
between  29.5  inches  and  28.5  inches  will  occur  by  taking  the  propor- 
tional distances.  The  air  pressure  29.4  will  occur  on  the  circle 
drawn  around  A  at  I  of  the  distance  towards  C;  the  air  pressure 


3  4 

FIG.  24.  —  ILLUSTRATION  OF  LOCAL  INCREASE  AND  DECREASE  OF  AIR  PRESSURE. 

29.3,  at  another  |  of  this  distance,  etc. :  so  that,  if  we  wish  to  show  the 
location  of  the  pressures  for  each  -^  of  an  inch,  we  should  have  a  dia- 
gram like  Fig.  24  (4). 

The  numerals  on  these  circles  show  the  air  pressure  on  this  plane ; 
and  they,  with  the  position  of  the  circles,  show  us  the  relative  loca- 


88  ELEMENTARY   METEOROLOGY. 

tions,  above  and  below  this  plane,  of  the  isobaric  surfaces  which  inter- 
sect it  at  these  circles  ;  and  this  is  the  best  graphical  presentation  that 
we  can  give  of  them,  because  it  is  so  difficult  to  represent,  on  a  sheet 
of  paper  having  only  length  and  breadth,  the  additional  feature  of 
thickness. 

It  is  by  a  similar  graphical  process  that  the  air  pressures  over  the 
earth  are  shown  on  charts.  We  trace  out  by  isobaric  lines  the  inter- 
section of  the  isobaric  surfaces  with  some  chosen  level  or  plane,  that  of 
the  sea  level  being  usually  selected ;  and  by  so  locating  these  isobaric 
surfaces  as  shown  by  observations,  we  can  determine  the  regions  of 
greatest  and  least  air  pressure. 

Geographical  Distribution  of  Air  Pressure.  —  In  compar- 
ing the  average  air  pressures  at  various  places,  the  observed 
pressures,  as  we  have  seen,  must  be  reduced  to  a  common 
level.  It  is  usual  to  make  the  reduction  to  the  sea  level ; 
but  any  level  may  be  chosen,  and,  as  we  shall  see,  other 
levels,  are  also  sometimes  used  as  a  plane  of  reduction. 

Charts  have  been  prepared,  showing  the  distribution, 
over  the  surface  of  the  earth,  of  the  average  air  pressure 
(reduced  to  sea  level)  for  each  month  of  the  year.  The 
isobaric  lines  or  isobars  drawn  on  these  charts  show  very 
clearly  the  regions  of  relatively  high  and  low  air  pressures, 
and  the  intermediate  conditions. 

Areas  of  relatively  high  air  pressure  are  called  barometric 
maxima,  or  sometimes  simply  highs;  and  areas  of  rela- 
tively low  pressure,  barometric  minima,  or  simply  lows. 

Only  selected  charts  are  given  here  for  the  year,  and  for  the  months 
of  January  and  July,  on  which  are  indicated  the  isobaric  lines  which 
show  where  the  isobaric  surfaces  cut  the  plane  of  the  sea  level.  The 
order  of  the  distribution  of  the  air  or  barometric  pressure  depends  on 
two  great  causes,  —  primarily  on  the  air  temperatures,  as  shown  by 
their  distribution  over  the  surface  of  the  earth  ;  and  secondarily  on 
the  movement  of  the  air  itself  from  one  part  of  the  earth's  surface 
to  another.  The  discussion  of  the  relation  of  these  causes  to  their 


90  ELEMENTARY    METEOROLOGY. 

effects  must  be  deferred  until  the  movements  of  the  atmosphere  are 
presented  in  a  later  chapter.  The  mere  facts  of  the  distribution  of  the 
air  pressure  are  all  that  can  be  given  at  present. 

Distribution  of  the  Average  Air  Pressure  for  the  Year.  — 
This  chart  of  isobaric  lines  at  sea  level  for  the  year  (Fig., 
25)  shows,  in  the  northern  hemisphere,  an  area  of  high  air 
pressure  of  more  than  30.1  inches  over  eastern  Asia; 
another  with  a  pressure  of  more  than  30.2  inches  over  the 
eastern  North  Pacific  Ocean,  in  the  same  latitude ;  and 
another  of  more  than  30  inches  over  North  America.  In 
the  southern  hemisphere  the  regions  of  high  pressure  are, 
over  the  Atlantic  Ocean,  near  latitude  25°,  more  than  30.1 
inches ;  over  the  Indian  Ocean,  west  of  Australia,  more 
than  30.1  inches;  over  the  eastern  Pacific  Ocean,  between 
latitude  30°  and  40°,  more  than  30.2  inches. 

Areas  of  low  air  pressure  are  found,  for  the  northern 
hemisphere,  over  the  North  Atlantic  Ocean,  near  Iceland, 
with  pressure  below  29.7  inches;  over  the  extreme  north- 
ern Pacific  Ocean,  with  pressure  below  29.7  inches;  and 
to  the  southeast  and  southwest  and  over  the  southern  part 
of  Asia,  with  pressure  below  29.8  inches.  In  the  southern 
hemisphere  the  regions  of  low  air  pressure  are  over  north- 
ern Australia  and  over  the  Antarctic  Ocean,  where  there 
is  a  quite  uniform  decrease  from  30  inches  just  north  of 
latitude  40°,  to  29.3  inches  in  latitude  60°. 

Distribution  of  the  Average  Air  Pressure  for  January.  — 
The  chart  of  the  isobaric  lines  at  sea  level,  for  the 
month  of  January  (Fig.  26),  shows  that  in  the  northern 
hemisphere  there  is  an  area  of  high  barometric  pressure 
over  central  Asia  of  more  than  30.5  inches,  and  over 
central  North  America  of  more  than  30.2  inches;  in  both 
cases  lying  mainly  north  of  the  4Oth  parallel.  In  the 
southern  hemisphere  the  areas  of  maximum  pressure  of 


92  ELEMENTARY   METEOROLOGY. 

30.1  inches  are  on  the  equatorial  side  of  the  4Oth  parallel 
over  the  oceans  to  the  westward  of  Australia,  Africa,  and 
South  America. 

Areas  of  lowest  pressure  in  the  northern  hemisphere 
are,  over  the  North  Atlantic,  near  Iceland,  less  than  29.5 
inches ;  and  over  the  central  North  Pacific,  less  than  29.6 
inches.  In  the  southern  hemisphere  the  areas  of  lowest 
pressure  are,  over  northern  Australia,  less  than  29.7 
inches ;  the  Indian  Ocean,  less  than  29.8  inches ;  and 
southern  Africa,  29.8  inches.  South  of  the  4Oth  parallel 
there  is  a  rapid  but  regular  decrease  from  29.9  inches,  to 
about  29  inches  in  the  Antarctic  latitudes. 

The  distribution  of  the  average  air  pressure  for  January  reduced  to 
planes  above  the  sea  level  (as  indicated  by  isobaric  lines  drawn  for 
other  elevations  than  that  of  the  sea  level  as  just  given)  shows  that  for 
the  January  average  air  pressures  the  above-mentioned  maximum  at  the 
earth's  surface  over  Asia  exists  only  for  the  lower  air  layers ;  and  at  an 
altitude  of  about  5,000  feet,  the  North  American  and  European  maxi- 
mum air  pressures  disappear.  At  an  altitude  of  about  10,000  feet,  the 
maximum  air  pressure  lies  over  the  equatorial  region,  and  it  decreases 
thence  towards  the  poles. 

The  distribution  of  air  pressure  for  January,  at  an  altitude  of  13.000 
feet,  is  about  as  follows  :  — 

Over  the  continents  in  the  northern  hemisphere  there  is  a  region  of 
low  pressure  over  northern  North  America  (16.8  inches)  and  Asia 
(16.5  inches)  ;  but  on  the  intervening  oceans  the  pressure  is  higher, 
especially  over  the  North  Atlantic  Ocean  (17.3  inches).  From  these 
northern  regions  there  is  an  increase  with  southward  progress,  more 
rapid  over  the  land  than  over  the  sea,  down  to  the  region  between 
the  equator  and  the  Tropic  of  Capricorn,  where  a  maximum  pressure 
of  about  18.7  inches  is  reached.  Within  this  maximum  region  the 
pressures  vary  somewhat,  and  are  a  little  less  over  the  Atlantic  Ocean 
than  elsewhere.  From  this  region  of  high  pressure  there  is  a  decrease 
towards  the  south,  and  a  pressure  of  about  17.7  inches  is  reached  at 
latitude  50°  south. 

It  is  to  be  particularly  noticed  that  the  courses  of  the  isobars  are 


94  ELEMENTARY    METEOROLOGY. 

very  irregular  over  the  northern  hemisphere,  where  the  earth's  surface 
is  mainly  land,  and  that  they  run  regularly  along  the  parallels  of  lati- 
tude in  the  southern  hemisphere,  where  the  surface  is  mostly  water. 

Distribution  of  Air  Pressure  at  Sea  Level  for  July  (Fig. 
27).  — The  areas  of  high  pressure  for  July  in  the  northern 
hemisphere  lie  north  of  the  Tropic  of  Cancer  (over  the 
Atlantic  Ocean,  30.2  inches;  and  over  the  Pacific  Ocean, 
30.3  inches).  In  the  southern  hemisphere  the  areas  of 
high  pressure  lie  over  the  oceans,  south  of  the  Tropic  of 
Capricorn  (in  the  eastern  Pacific  and  mid-Atlantic,  30.2 
inches;  and  in  the  western  Indian  Ocean,  30.3  inches). 

The  areas  of  low  pressure  in  the  northern  hemisphere 
lie  over  the  continents,  a  little  north  of  the  Tropic  of 
Cancer  (in  western  Asia,  29.4  inches ;  in  western  America, 
29.8  inches).  There  is  also  an  area  of  low  pressure  over 
the  Atlantic  near  Iceland  (29.8  inches).  In  the  southern 
hemisphere,  south  of  latitude  40°,  the  pressure  is  below  30 
inches,  and  decreases  rapidly  towards  the  Antarctic  regions, 
being  29.4  inches  in  latitude  60°, 

For  July,  at  the  higher  planes  above  the  sea  level,  the  distribution 
of  the  air  pressure  differs  from  that  just  described.  At  about  5,000  feet 
altitude,  the  curves  are  much  more  regular  than  at  sea  level.  At  about 
lo.ooo  feet  altitude,  the  zone  of  the  highest  pressure  approaches  the 
equator,  and  at  13,000  feet  it  reaches  the  equatorial  region. 

The  distribution  of  air  pressure  for  July,  at  an  altitude  of  13.000 
feet,  is  as  follows  :  — 

The  region  of  highest  pressure  lies  mainly  on  the  north  side  of  the 
equator,  and  extends  even  beyond  the  Tropic  of  Cancer.  This  region 
of  maximum  pressure  is  somewhat  broken ;  and  over  the  center  of 
Africa,  southern  North  America,  the  Middle  Atlantic  and  Pacific 
oceans,  the  pressure  reaches  maximum  values  of  18.6  inches  or  18.7 
inches,  which  is  slightly  above  that  of  the  intervening  regions  along 
the  Tropic  of  Cancer.  To  the  northward  the  pressure  decreases  to 
17.8  inches  over  northern  North  America,  and  17.9  inches  over  the 
North  Atlantic  and  northern  Asia ;  but  the  decrease  is  more  gradual 


AIR  PRESSURE.  95 

over  the  western  coasts  than  over  the  eastern  coasts.  To  the  south- 
ward of  the  equator  the  decrease  is  much  more  regular  than  towards 
the  north,  and  at  about  latitude  55°  south  the  pressure  (17.6  inches) 
is  without  many  irregularities  with  change  of  longitude ;  which  shows, 
that,  even  at  this  altitude,  the  difference  between  land  and  water  sur- 
faces is  noticeable. 

Average  Air  Pressure  along  a  Meridian.  —  The  average 
change  of  air  pressure  with  latitude,  or  the  change  along 
an  average  meridian,  may  be  seen  in  the  table  on  p.  96, 
which  gives  the  average  or  normal  pressure  for  each  5° 
of  latitude  for  the  year,  January,  and  July,  at  sea  level; 
and  for  the  year  at  altitudes  of  about  6,500  feet  and 
13,000  feet. 

For  the  year,  at  the  sea  level,  there  is  a  maximum  air 
pressure  at  about  latitude  35°  in  the  northern  hemisphere, 
and  30°  in  the  southern ;  with  a  decrease  on  both  sides 
towards  the  equator  and  towards  the  poles.  This  pole- 
ward decrease  is  very  much  greater  in  the  southern  hemi- 
sphere than  in  the  northern,  due  to  the  effects  of  the 
uniform  water  surface  in  the  former;  and  in  fact  in  the 
northern  hemisphere  there  is  a  slight  increase  again  at  the 
far  north.  The  decrease  toward  the  equator  is  very  slight. 

At  6,500  feet  altitude  above  the  sea  level,  the  place  of 
maximum  pressure  in  each  hemisphere  approaches  the 
equator,  and  is  quite  uniform  near  it,  and  the  decrease 
towards  the  poles  is  much  more  nearly  equal  in  the  two 
hemispheres. 

At  13,000  feet  altitude,  a  single  maximum  occurs  near  the 
equator  (a  little  to  the  south  of  it),  and  the  diminution  of 
the  pressure  towards  the  poles  is  still  more  symmetrical 
for  the  two  hemispheres.  The  area  of  high  pressure  at  the 
north  pole  at  sea  level  seems  to  disappear  at  a  slight  altitude, 
just  as  it  does  in  the  more  local  region  of  eastern  Siberia. 


96 


ELEMENTARY   METEOROLOGY. 


AVERAGE  AIR  PRESSURES  ALONG  A  MERIDIAN. 


AVERAGE  AIR  PRESSURE  AT 
SEA  LEVEL. 

AVERAGE  AIR  PRESSURE 
FOR  THE  YEAR  AT 
ALTITUDE  OF 

LATITUDE. 

YEAR. 

JAN. 

JULY. 

6,562    FT. 

13,^23    FT. 

Inches. 

Inches. 

Inches. 

Inches. 

Inches. 

North       80°     ... 

29.941 

29-937 

29-945 

22.913 

17.528 

75°     ... 

29.921 

29.929 

29.874 

70°     ... 

29.866 

29.882 

29.850 

22.976 

17.583 

65°     ... 

29.850 

29.874 

29.827 

60°     ... 

29.870 

29.910 

29.831 

23-I34 

17.791 

55°    ..- 

29.910 

29.961 

29.858 

50°    .     -     - 

29.949 

30.004 

29.894 

23-347 

17.992 

45°     ... 

29.980 

30.039 

29.921 

40°     ... 

30.000 

30.063 

29.937 

23-543 

18.252 

35°     -.. 

30.016 

30.083 

29.949 

30°     ... 

29.988 

30.055 

29.921 

23-658 

18.437 

25"     ... 

29.937 

30.000 

29.874 

20°      ... 

29.890 

29.945 

29.835 

23-658 

18.500 

15°      -.. 

29.854 

29.894 

29.815 

10°      .       .       . 

29.839 

29.858 

29.819 

23-658 

18.532 

North        5°     ... 

29-843 

29.843 

29.839 

Equator     o°     . 

29.843 

29.819 

29.866 

23-665 

18.543 

South        5°     ... 

29.854 

29.807 

29.902 

10°      .       .       . 

29.886 

29.819 

29.953 

23-685 

18-547 

15°     •     •     • 

29.929 

29.850 

30.008 

20°      ... 

29.988 

29.902 

30-075 

23.728 

18-547 

25°      ... 

30.047 

29-953 

30.154 

30°      ... 

30-059 

29-973 

30.146 

23.709 

18.476 

35°     •     •     • 

30.016 

29-945 

30.087 

40-    ... 

29.941 

29.886 

29.996 

23-508 

18.232 

45°     -.. 

29.815 

29.776 

29.854 

50°     ... 

29.654 

29.634 

29.673 

23-150* 

17.862 

55°     ... 

29-457 

29-457 

29-457 

60°    ... 

29.268 

— 

— 

22.717 

17.476 

65°    ... 

29.122 

— 

— 

— 

— 

South       70°     .     .     . 

29-055                 — 

— 

— 

— 

AIR   PRESSURE. 


97 


The  accompanying  figure  (Fig.  28)  shows  the  general  slope  of  the 
isobaric  surfaces  along  a  meridian,  at  sea  level  and  at  various  altitudes 
above  it.  The  dotted  line  shows  where  the  isobaric  surface  of  30 
inches  lies  below  the  sea  level,  the  pressure  at  that  level  being  less 
than  30  inches. 


Attitude  6,500  feet____ 
23.5  inches' 


Altitude  0,2,000  feet 


N.Latitude 


Sea 


Level 


Isobaric  Surface 


20'  0°  20°  40°  •»*".-_    80° 

FIG.  28.  —  SLOPE  OF  ISOBARIC  SURFACES  ALONG  A  MERIDIAN  AT  VARIOUS  ALTITUDES. 

The  Variability  of  the  Average  Air  Pressure  increases 
with  the  latitude,  and  to  a  certain  extent  with  the  increase 
of  the  distance  inland.  With  increase  of  altitude  above 
the  sea  level,  the  variability  decreases  in  winter,  but  in- 
creases in  the  summer  for  low  altitudes.  The  variability  is 
greater  in  winter  than  in  summer. 

Long-period  Oscillations  of  Air  Pressure,  extending  over 
a  period  of  a  little  more  than  30  years,  undoubtedly  exist ; 
but  the  extreme  average  maximum  pressure  for  a  number 
of  places  is  not  more  than  0.06  of  an  inch  greater  than 
the  extreme  average  minimum.  Slight  as  these  variations 
are,  they  are  sufficient  to  cause  great  climatic  oscillations, 
owing  to  their  effects  on  the  atmospheric  circulation.  It  is 
very  probable  that  when  there  is  an  excess  of  air  pressure 
over  the  continents,  there  is  a  deficiency  over  the  oceans. 

Physiological  Effects  of  Air  Pressure.  —  The  outside  sur- 
face of  the  body  exposed  to  the  air  pressure,  in  the  case  of 
a  grown  person,  is  about  16  square  feet,  and  this  would 

WALDO    METEOR.  —  6 


98  ELEMENTARY   METEOROLOGY. 

give  a  pressure  of  about  35,000  pounds.  If  it  were  not 
for  the  ease  with  which  this  air  penetrates  the  body  under 
this  pressure,  very  slight  changes  in  it  would  prove  dis- 
astrous to  life.  The  variations  in  this  pressure  may  arise 
in  two  ways,  — first,  through  the  changes  in  the  air  pressure 
at  the  same  altitude  (due  to  the  passage  of  areas  of  high 
and  low  barometric  pressure,  in  which  the  extreme  cases 
would  give  a  variation  of  not  over  4  inches)  from  about 
27.5  inches  to  31.5  inches;  second,  through  the  change  in 
altitude  and  increase  in  pressure  by  making  a  descent,  or 
decrease  in  pressure  by  an  ascent.  In  the  former  case  of 
varying  the  pressure  merely,  a  maximum  change  of  O.8 
of  an  inch  air  pressure  in  a  day  would  be  equivalent  to 
changing  the  altitude  by  about  690  feet  during  that  time ; 
or  in  the  extreme  case  of  an  absolute  change  from  31.5 
inches  to  27.5  inches  air  pressure,  it  would  be  the  same  as 
coming  from  a  depth  of  about  1,378  feet  below  the  ground 
surface,  and  ascending  to  a  height  of  about  2,133  ^eet 
above  this  surface.  In  other  words,  the  changes  in  air 
pressure  which  we  may  experience  at  the  earth's  surface 
are  as  great  as  those  which  would  be  encountered  in  an 
ascent  of  about  3,500  feet  in  altitude. 

There  are,  however,  limits  to  the  amount  that  the  air  pressure  can 
be  changed  without  harm.  Just  what  these  limits  are,  we  do  not  know, 
and  they  would  vary  for  different  persons ;  but  one  individual  (Hum- 
boldt)  was  exposed  to  a  barometric  pressure  of  about  48  inches  in  a 
diving  bell,  and  to  14.75  inches  on  a  mountain  top;  others  have  been 
exposed  to  a  still  lower  air  pressure. 

It  is  probable  that  animals  can  stand  a  somewhat  greater  increase 
than  decrease  of  pressure.  In  cases  of  such  extreme  variations  of  air 
pressure  the  changes  must  take  place  gradually.  If  they  take  place  too 
rapidly,  fainting,  bursting  of  blood  vessels,  or  even  death,  will  result. 
If  the  change  takes  place  slowly,  then,  as  the  limit  of  endurance  is 
approached,  certain  physiological  effects  are  made  apparent.  These 


AIR   PRESSURE.  99 

are  the  symptoms  of  the  so-called  mountain  sickness,  such  as  difficulty 
of  breathing,  headache,  pressure  on  the  eardrums,  and  a  general  apathy, 
which  prevents  exertion  of  any  kind.  These  symptoms  disappear, 
however,  when  lower  altitudes  are  again  reached.  Permanent  residence 
at  very  high  altitudes  does,  however,  impoverish  the  blood  by  decreas- 
ing the  amount  of  oxygen  inhaled  in  breathing. 

Classified  Distribution  of  the  Air  Pressures. — The  whole 
lower  air  mass  may  be  divided  into  regions  of  low  and 
high  air  pressure,  or  areas  of  deficiency  or  excess,  between 
which  lie  areas  of  more  nearly  normal  pressure. 

Regions  of  Low  Barometric  Pressure  may  be  divided  into 
four  classes  or  orders  :  — 

1.  Depressions  of  the  first  order  are  those  belonging  to 
the  air  system  of  whole  hemispheres.     They  include  those 
permanent  ones  extending  equatorward  and  poleward  from 
about  latitude  30°.     They  are  of  permanent  character. 

2.  Depressions  of  the  second  order  are  of  great  extent, 
covering  areas  many  hundreds  of  miles  in  diameter,  and 
occur  mainly  in  the  middle  and  lower  latitudes.     They  are 
not  permanent  as  regards  either  location  or  duration. 

3.  Depressions  of  the  third  order  are  minor  depressions 
occurring  within  depressions  of  the  second  order. 

4.  Depressions  of  the  fourth  order  are  those  arising  from 
the  local  excess  of  temperature.     They  are  fixed  in  posi- 
tion, but  are  not  permanent.      Such  occur  during  the  day- 
time over  limited  isolated  land  areas  (as,  for  instance,  the 
Spanish  Peninsula). 

Regions  of  High  Barometric  Pressure  may  be  divided  into 
three  classes  or  orders,  as  follows  :  — 

I.  Regions  of  high  pressure  of  the  first  order  are  the 
permanent  hemispherical  ones  to  be  found  in  the  lower, 
middle,  and  north  polar  latitudes.  They  extend  upwards 
to  considerable  altitudes. 


100  ELEMENTARY   METEOROLOGY. 

2.  Regions   of   high   pressure  of  the  second  order  are 
those  resulting  from  the  heaping-up  of  air  between  two 
depressions  of  the  second  order.     They  are  therefore  not 
permanent  in  either  duration  or  location. 

3.  Regions  of  high  pressure  of  the  third  order  are  caused 
by  the  local  cooling  of  the  air  at  the  earth's  surface.     They 
are  of  local  occurrence,  and  do  not  extend  to  great  altitudes. 

Barometric  Gradient.  —  By  gradient  is  usually  meant  the 
degree  of  steepness  of  a  slope.  In  meteorology  we  speak 
of  the  steepness  of  the  slope  of  isobaric  surfaces  as  the 
barometric  gradient.  Instead  of  measuring  the  angle  of 
the  slope,  it  is  customary  to  take  the  difference  in  baro- 
metric pressure  along  the  same  level.  As  a  unit  of 
distance  in  all  directions,  the  length  of  a  degree  of 
the  meridian  is  taken.  The  barometric  gradient  between 
two  places  is,  then,  the  difference  in  the  barometric  pres- 
sures at  the  same  level,  divided  by  the  number  of  merid- 
ional degrees  between  the  two  places.  Thus,  if  at  one 
place  the  barometric  pressure  is  30  inches,  and  at  another 
place  the  pressure  (reduced  to  the  same  level)  is  30.50 
inches,  and  the  distance  between  the  two  places  is  five 
meridional  degrees,  then  the  barometric  gradient  between 
the  two  places  would  be  (30. 5  —  30)  -5-  5,  or  o.  I,  of  an  inch. 

Wherever  a  gradient  exists,  there  is  a  gradient  force 
acting,  due  to  the  efforts  of  gravity  to  render  the  isobaric 
surfaces  level.  The  amount  of  this  gradient  force,  in 
the  case  of  isobaric  surfaces,  increases  with  increase  of  the 
magnitude  of  the  gradient ;  that  is,  the  steeper  the  incli- 
nation of  the  isobaric  surfaces,  the  greater  the  gradient 
force.  This  gradient  force  is  effective,  or  operates,  in  the 
direction  of  the  slope  of  the  isobaric  surfaces,  and  air  flows 
down  a  sloping  isobaric  surface  in  a  manner  somewhat 
similar  to  the  flowing  of  water  down  an  incline. 


CHAPTER   IV. 
WINDS 

Air  Currents.  —  When  the  air  is  at  rest,  relative  to  sur- 
rounding objects,  we  are  scarcely  aware  of  its  existence; 
but  when  it  is  in  motion,  we  notice  that  it  exerts  a  pressure 
or  force  against  these  objects.  Air  in  motion  is  called 
wind.  When  air  is  at  rest,  it  is  said  to  be  calm.  The  air 
of  the  atmosphere  would  as  a  whole  be  continually  calm, 
if  it  were  not  for  the  unequal  distribution  of  temperature 
over  the  earth's  surface  and  through  the  air  itself.  This 
unequal  heating  gives  rise  to  air  motion  or  air  currents, 
the  passage  of  which  gives  the  phenomenon  of  wind. 

The  air  in  moving  must  go  from  some  one  place  to 
another ;  and  therefore  wind  has  direction,  as  we  see  from 
the  movement  of  light  objects,  such  as  leaves  or  clouds, 
which  are  carried  along  by  the  wind.  There  must  also 
be  a  rate  of  motion,  which  we  call  wind  velocity ;  and  we 
judge  of  this  by  the  rapidity  with  which  the  air  transports 
such  light  objects.  Heavy  objects  lag  behind  the  true 
motion  of  the  air. 

Since  air  has  density  or  mass,  then  when  it  is  in  motion 
it  must  exert  a  pressure  or  force,  called  wind  force  or  wind 
pressure,  against  objects  with  which  it  comes  in  contact, 
as  is  realized  by  the  way  the  wind  bends  trees,  or  blows 
against  our  bodies. 

Air  currents  moving  approximately  parallel  to  the  sur- 
face of  the  ocean  are  called  horizontal  air  currents.  Air 

101 


102  ELEMENTARY   METEOROLOGY. 

currents  moving  approximately  perpendicular  to  the  sur- 
face of  the  ocean  are  called  vertical  air  currents.  The 
possible  movement  of  vertical  currents  is  limited  to  a  few 
miles  between  the  earth's  surface  and  the  outer  limit  of 
the  air,  while  horizontal  air  currents  might  make  the  entire 
circuit  of  the  earth.  The  usual  direction  .of  the  wind  is 
some  combination  of  these  two  directions.  The  vertical 
movement  of  the  air  is  usually  so  slight,  as  compared 
with  the  horizontal  movement,  that  ordinarily  the  latter 
only  is  observed  for  general  winds. 

The  Winds  of  the  Globe  may  be  divided  into  the  follow- 
ing classes :  — 

1.  A  permanent  and  continual  interchange  of  air  be- 
tween the  equatorial  and  polar  regions,  due  to  the  differ- 
ences   in    temperature    between    those    regions :    this    is 
called   the  general  circulation   of   the   atmosphere.      The 
annual    shifting   of   the   thermal    equator  (which    follows 
the  latitude  of  the  sun)  causes  an  annual  displacement 
and  inequality  of  this  system  of  winds. 

2.  An  interchange  of  air  between  the  masses  lying  over 
bodies  of  water  and   of  land,  due  to  the   unequal   heat- 
ing of   the   two  surfaces.     When  the   interchange  takes 
place   over    great    regions,    as    between    continents    and 
oceans,  it  has    an    annual  period  of    occurrence :   this  is 
called  the  monsoon  wind.     When  the  interchange  takes 
place  between  the  coast  lands  and  the  coast  waters  only, 
it  is  diurnal  in  occurrence  :  this  is  called  the  land  and  sea 
breeze. 

3.  An  interchange  of  air  between  mountains  and  val- 
leys, due  to  the  unequal  heating  and  cooling  of  the  two 
localities :    this  is  called  the   mountain  and  valley  breeze, 
and  is  diurnal  in  its  occurrence. 

4.  An  interchange  of  air  between  extensive  regions  of 


WINDS.  103 

higher  and  lower  barometric  air  pressures  :  this  gives  rise 
to  the  cyclonic  and  anticyclonic  winds,  which  are  irregular 
in  occurrence. 

5.  A  local  rush  of  air  for  the  restoration  of  the  normal 
condition  (stable  equilibrium)  when  the  latter  has  been  dis- 
turbed by  local  causes :   to  this  class  belong  spouts,  torna- 
does, and  squalls,  which  are  of  occasional  occurrence, 

6.  A  rush  of  air  which  is  produced  in  front  of  ava- 
lanches and  land  slides :  this  is  called  avalanche  wind,  and 
is  of  occasional  occurrence. 

7.  The  outrush  of  air  which  takes  place  with  volcanic 
eruptions  :    this  may  be  called  volcanic  wind,  and  is  of 
occasional  occurrence. 

8.  To   these   must  be  added   another   class  of   winds, 
which  may  be  called  eddy  winds  or  whirlwinds.      These 
occur  as  follows :  When  a  current  of  air  is  flowing  steadily 
along,  there  break  out  at  intervals  from  the  sides  whirls 
or  eddies  such  as  may  be  seen  in  flowing  water ;  or  they 
may  be  produced  by  the  meeting  of  two  air  currents  differ- 
ing in  direction.     These  are  irregular  in  occurrence,  and 
vary  in  magnitude  from  large  whirls  produced  by  the  gen- 
eral air  currents  (No.  i)  to  the  small  dust  whirls  which  we 
see  in  the  streets. 

Direction  of  the  Wind.  —  The  direction  of  the  wind  is 
named  from  the  direction  of  approach.  Thus,  if  a  wind 
blows  from  the  north,  it  is  said  to  be  a  north  wind ;  if  it 
blows  from  the  east,  it  is  said  to  be  an  east  wind ;  etc. 

In  accurate  work  in  meteorology,  16  points  of  compass  are  used,  as 
follows :  — 

North,  north  northeast,  northeast,  east  northeast,  east,  east  south- 
east, southeast,  south  southeast,  south,  south  southwest,  southwest, 
west  southwest,  west,  west  northwest,  northwest,  and  north  northwest. 
These  are  indicated  by  their  initial  letters,  as  shown  in  Fig.  29. 


104 


ELEMENTARY    METEOROLOGY. 


N. 


The  Horizontal  Direction  of  the  Wind  is  usually  observed 

by  means  of  an  instrument  called  the  arrow  wind  vane 

(Fig.  30),  the  head  of  the  arrow  pointing  in  the  direction 

of  approach  of  the  wind. 
The  arrow  is  free  to  re- 
volve horizontally  around 
a  vertical  axis  placed  near 
the  head.  The  best  vanes 
have  a  divided  tail  with  a 
divergence  of  about  22°. 

The  Velocity  of  the  Wind 
is  the  rapidity  with  which 
the  air  moves  past  some 
fixed  point,  or  covers  the 
known  distance  between 
some  two  points.  Wind 

velocities  are  usually  given  in  miles  per  hour,  or  feet  per 

second,  in  English  measure. 

The  Anemometer.  —  An  instrument  used  to  measure  the 

velocity   of   the    wind    is 

called  an  anemometer.     It 

is    a    small    wind    wheel 


FIG.  29. —POINTS  OF  COMPASS  FOR  WIND 
DIRECTIONS. 


Vertical  Sec. 


FIG.  30.  — ARROW  WIND  VANE. 


with  an  attachment  by  ~T^— — Horizontal  sec. 
means  of  which  the  num- 
ber of  revolutions  of  the 
wheel  is  indicated  on  a 
little  dial.  When  exposed  to  the  wind,  the  rapidity  of 
revolution  of  the  wind  wheel  varies  with  the  actual  veloc- 
ity of  the  wind.  The  commonest  form  is  the  Robinson 
cup  anemometer  shown  in  Fig.  31. 

The  four  hemispherical  cups  revolve  as  a  whole  around  a  vertical 
axis,  when  exposed  to  the  wind.  It  has  been  found  that  the  distance 
traveled  by  any  one  of  the  cups,  when  multiplied  by  about  2.5,  will  give 


WINDS. 


105 


the  velocity  of  the  wind.  The  distance  passed  over  in  one  revolution 
of  the  cup  is  found  by  multiplying  the  distance  from  the  center  of  the 
vertical  axis  to  the  center  of  the  cup  by  (2)  x  (3.1416).  The  dial  on 
the  vertical  axis  shows  the  number  of  miles  of  wind. 

The  Force  of  the  Wind  is  .the  pressure  which  it  exerts 
on  a  flat  surface  held  perpendicular  to  its  direction  of  mo- 
tion.    It  may  be  judged 
by   an    observer,    or   ex- 
pressed in  actual  pounds 
of    pressure    per   square 
foot  of  exposed  surface. 

Observation  of  Wind 
Direction. — In  most  cases 
when  wind  direction  is 
observed  two  or  three 
times  daily,  as  in  ordi- 
nary observations,  the 
results  are  tabulated  for 
each  month  and  for  the 
year  by  giving  the  num- 
ber of  times  the  wind 
blows  from  each  of  the 
eight  principal  points  of  the  compass,  and  the  number  of 
calms  during  the  time. 

Thus  in  1891  the  number  of  times  the  wind  blew  from  various  direc- 
tions at  St.  Paul  during  the  year  and  the  extreme  months,  as  shown  by 
two  observations  daily,  was  as  follows  :  — 


FIG.  31.  —  ROBINSON  CUP  ANEMOMETER. 


N. 

N.  E. 

E. 

S.  E. 

S. 

S.W. 

w. 

N.  W. 

Calm. 

January      „     .     .     . 
July  
Year       .... 

2 

3 

°4. 

O 

4 

-JQ 

2 

8 

1:7 

23 
19 
224 

3 

AT. 

4 

9 
oo 

7 
7 
84. 

17- 
9 
1  1  1 

4 
2 

c6 

ou 

106  ELEMENTARY   METEOROLOGY. 

The  direction  from  which  the  winds  blow  with  greatest 
frequency  is  called  the  direction  of  prevailing  winds.  It 
varies  for  different  regions. 

Tables  have  been  published  which  give  the  relative  frequency  of  the 
wind  from  the  different  points  of  compass,  for  the  months  and  the  year, 
for  various  regions  of  the  earth ;  and  charts  have  been  drawn  which 
give  the  direction  of  the  prevailing  winds,  for  each  month  and  the 
year,  for  portions  of  the  continents  and  traversed  seas  of  the  globe. 
(See  wind  directions  given  on  the  charts,  Figs.  26/27,  showing  the 
distribution  of  air  pressure  at  sea  level  over  the  whole  earth.  The 
arrows  fly  with  the  wind.) 

Where  the  wind  blows  at  different  times  in  different 
directions,  all  of  these  motions  combined  are  equivalent  to 
a  motion  in  a  single  direction,  called  a  resultant  direction. 

This  may  be  explained  graphically  as  follows :  If  we  start  at  a  point 
A,  and  draw  a  line  in  the  proper  direction  equal  to  the  amount  of  north 
wind,  and  from  this  end  of  the  line  draw  another  line  in  the  proper 
direction  equal  to  the  amount  of  the -northeast  wind,  and  from  the  end 
of  this  line  lay  off  in  the  proper  direction  another  line  equal  to  the 
amount  of  the  east  wind,  and  so  on  for  all  the  wind  directions,  then 
a  line  drawn  from  A  to  the  end  of  the  last  line  drawn,  will  represent 
the  direction,  and  in  length  the  amount,  of  the  resultant  wind. 

Observation  of  Wind  Velocities.  —  Wind  velocities  are 
subject  to  diurnal  and  annual  periodic  variations  and  to 
marked  irregular  changes.  The  diurnal  change  of  wind 
velocities  has  in  general  but  a  single  maximum  and  mini- 
mum. The  maximum  usually  occurs  at  about  two  to  three 
hours  after  noon  for  the  average  of  the  year  on  the  land  near 
the  surface  of  the  ground,  and  about  the  same  time  before 
noon  on  the  ocean ;  and  the  hour  of  minimum  varies  from 
two  to  eight  hours  after  midnight  on  the  land,  but  is 
nearer  midnight  (it  may  be  before  or  after)  on  the  ocean. 
The  diurnal  amplitude  or  difference  between  the  maximum 
and  minimum  wind  is  small  over  the  ocean,  but  over  the 


WINDS. 


107 


land  it  is  large.     In  the  United  States  it  varies  from  20 
to  1 20  per  cent  of  the  average  velocity  of  the  wind. 

The  daily  variations  in  the  wind  velocity  at  considerable 
altitudes  above  the  ground  are  the  reverse  of  those  at 
the  earth's  surface  as  to  time  of  occurrence  of  principal 
phases.  From  observations  which  have  been  made,  it 
appears,  that,  at  an  altitude  of  perhaps  400  to  600  feet 
above  the  ground,  the  daily  change  in  the  wind  velocity 


FIG.  32.  — HOUR  OF  MAXIMUM  WIND,  IN  THE  UNITED  STATES,  FOR  JANUARY. 

practically  disappears,  while  at  greater  altitudes  occur 
phases  the  reverse  of  those  at  the  surface  of  the  ground, 
but  with  less  amplitudes.  In  the  warm  season  of  the  year 
the  maximum  and  minimum  phases  are  much  more  pro- 
nounced, and  the  time  of  maximum  is  later  in  the  after- 
noon than  in  the  colder  season. 

During  the  night  there  is  little  change  in  the  wind 
velocity ;  but  as  soon  as  the  sun's  heat  makes  itself  felt, 
there  is  a  more  or  less  rapid  increase  near  the  surface  of 
the  ground,  and  a  slighter  decrease  at  greater  altitudes. 


io8 


ELEMENTARY   METEOROLOGY, 


The  cause  of  this  has  been  explained  as  follows  :  The  wind  veloci- 
ties increase  with  the  altitude  above  the  ground  to  a  certain  extent. 
When  the  ground  is  heated  by  the  sun's  rays,  the  warm  air  rises,  and 
cooler  air  from  above  must  descend  to  take  its  place.  This  warm  air 
from  the  ground  surface  carries  with  it  the  lesser  (horizontal)  velocities 
at  the  surface,  and  mixing  with  the  upper  air  retards  its  horizontal 
movement ;  while-  the  descending  upper  air  takes  with  it  the  greater 
horizontal  velocities  at  the  higher  altitudes,  and  thus  accelerates  the 
horizontal  movement  of  the  lower  air  with  which  it  mixes.  Some  of 


FIG.  33.  —  HOUR  OF  MAXIMUM  WIND,  IN  THE  UNITED  STATES,  FOR  JULY. 

the  increased  velocity  of  the  lower  air  is  directly  due  to  the  increased 
air  movement  caused  by  the  warming  of  the  air  during  the  daytime, 
and  would  take  place  even  if  the  velocities  aloft  were  not  greater  than 
those  below. 

The  accompanying  charts  (Figs.  32,  33)  show  the  continental  dis- 
tribution of  the  hours  of  maximum  wind  for  January  and  July. 

Fig.  34  shows  the  daily  march  of  the  wind  velocities,  in  miles  per 
hour,  for  the  extreme  months  of  January  and  July  at  a  few  selected 
stations  in  the  United  States. 

Great  absolute  diurnal  variations  in  the  wind  velocity  occur  in  some 
localities  ;  as,  for  instance,  in  July  for  San  Francisco  and  Corpus  Christi 
(coast  stations),  and  Whipple  Barracks  (an  inland  station). 


WINDS. 


I09 


On  cloudy  days  the  daily  amplitude  of  wind  velocity  is  relatively 
small,  because  the  action  of  the  sun  is  weak. 


I 


i 


s  j,  January 

10    2    4    6     8    10  12  14  16 


JQ12    4     6    8  ?0  12  U  16  18  20  22  %T 


15 

/ 

•\ 

/^ 

^ 

12 

s 

^ 

11 

/ 

S 

Key  West  11. 
10 
9 

11 
10 
Corpus  Christi  9 
Texas 

7 
St.Paul  e 
5 
9 
8 
7 
6 
lilpple  Barracksb 
Arizona      ± 

3 

San  Francisco  7 
6 

10 

^ 

S 

S^ 

8 

,** 

"^, 

~\ 

x^-- 

1 

^- 

—  . 

^ 

7 

^ 

""^ 

j 

S. 

s 

,X 

^ 

6 

X, 

^N« 

x' 

'^-4 

^- 

^ 

14 

^ 

13 

12 

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-^* 

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11 

^ 

/ 

\ 

10 

\ 

^ 

^ 

9 

y 

\ 

8 

\ 

s 

~\ 

7 

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f 

v^ 

6 

\ 

^ 

^^^ 

-N 

5 

\J 

f 

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/ 

4 

7 

f 

ll 

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*^, 

-"• 

^v_ 

3 

10 

^—  ^ 

•> 

9 

/ 

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/ 

^ 

8 

/ 

I 

7 

/ 

\ 

\ 

6 

\ 

V 

5 

K^ 

/ 

S 

4 

f 

ITS 

\ 

"^ 

3 

•^ 

1 

16 

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<- 

2 

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y 

1 

15 

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1 

14 

13 

, 

^^ 

12 

1 

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K^ 

11 

i 

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1  —  ' 

f 

^^ 

10 

j 

9 

^ 

8 

\ 

/ 

7 

\ 

6 

\^ 

^ 

FIG.  34.  —  DIURNAL  CHANGES  IN  WIND  VELOCITIES  (MILES  PER  HOUR). 

The  Annual  March  of  the  Wind  Velocity  has  in  general 
but  a  single  maximum  (usually  during  the  cold  season) 
and  minimum  (in  the  warm  season). 


The    maximum    occurs    about    March,    and    the    minimum    about 
August,  in  most  of  the  United  States ;   but  in  some  portions  of  the 


1 10 


ELEMENTARY  METEOROLOGY. 


western  section  the  primary  maximum  occurs  about  April,  and  there 
is  a  slight  secondary  minimum  about  December.  In  most  of  Europe 
the  maximum  wind  occurs  in  a  winter  month., 


J  FM  AM  J  J  A»S  O  X  D 


St.Paul  7 
San  Francisco  - 


/ 

s 

\ 

/ 

\ 

/ 

\ 

/ 

,' 

N\ 

^' 

">, 

1 

.^^- 

N, 

•, 



\ 

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~t 

<•' 

N 

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.^, 

_, 

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s. 

v. 

L- 

The  Annual  Amplitude  of  Oscil- 
lation of  the  average  hourly  wind 
velocities  for  the  extreme  months 
is  variable  for  different  regions, 
amounting  to  over  10  miles  per 
hour  in  some  cases;  that  is,  the 
windiest  month  may  have  an  aver- 
age of  over  10  miles  per  hour  more 

FIG..  35- -ANNUAL     CHANGES     »  wind  than  the  Calmest  month. 

WIND  VELOCITIES  (MILES  PERHOUR). 

In  the  United  States  the  amplitude  is  about  4  or  5  miles  per  hour, 
but  on  the  northwestern  coast  it  is  1 1  miles  per  hour.  On  the  central 
Great  Plains  there  is  relatively  slight  variation  in  the  wind  velocity  for 
the  different  months  of  the  year. 

The  following  table  and  diagram  (Fig.  35)  show  the  average 
monthly  wind  velocities,  and  consequently  the  annual  march  of  the 
wind,  at  selected  places  in  the  United  States :  — 


AVERAGE  MONTHLY  WIND  VELOCITIES  (miles  per  hour) . 


J. 

F. 

M. 

A. 

M. 

J. 

J. 

A. 

S. 

O. 

N. 

D. 

Year 

Block  Island  

16.3 

16.0 

15-3 

134 

12.2 

u.6 

II.O 

II.O 

13.0 

14.0 

iS.5 

16.9 

13.8 

Key  West  

10.4 

10.2 

10.6 

9.8 

8.9 

7.6 

7-3 

74 

8.0 

10.7 

10.7 

10.7 

9-3 

St.  Paul  

7-i 

7-5 

8-3 

9.0 

8.5 

7-5 

6-5 

6.7 

7-7 

7-9 

74 

7.0 

7-6 

Dodge  City,  Kan.    .   . 

10.3 

10.5 

u.6 

13-3 

12.9 

I2.I 

II.  I 

10.4 

II.2 

10.7 

9-5 

9-7 

ii.  i 

Corpus  Christi,  Tex. 

10.4 

10.9 

I2.I 

13-7 

13-3 

12.3 

10.7 

10.7 

10.6 

10.5 

1  0.0 

IO.I 

"•3 

San  Francisco  .... 

6-7 

7-i 

8.4 

9-5 

n.8 

II.7 

II.  I 

1  1.8 

9.3 

94 

6.2 

6-5 

9.0 

Tatoosh  Island,  Wash. 

16.0 

14.4 

I2.7 

10.6 

10.3 

7-9 

7.6 

74 

IO.I 

11.7 

14.8 

iS-7 

u.6 

The  Average  Wind  Velocity  for  the  Whole  Year  has  not 
been  determined  at  enough  places  to  permit  the  show- 
ing of  lines  of  equal  wind  velocities  for  the  whole  earth. 
These  have  been  drawn,  however,  for  the  United  States 


WINDS.  1 1 1 

(p.  355)  and  for  the  Russian  Empire,  and  roughly  indicated 
for  some  portions  of  the  oceans. 

In  the  United  States  the  wind  velocities  are  greatest  on  the  coast, 
and  in  general  decrease  towards  the  interior ;  but  on  the  treeless 
Great  Plains  near  the  center  of  the  continent  there  is  an  increase  again 
to  nearly  the  same  velocities  as  are  found  along  the  low  shores  of  the 
ocean. 

In  Russian  Siberia  there  is  a  decrease  towards  the  center  of  the 
continent ;  but  there  is  no  central  region  of  increased  wind,  as  found  in 
the  United  States.  Whether  this  is  due  entirely  to  the  difference  in  the 
physical  features  of  the  two  regions,  or  is  partly  due  to  the  higher  lati- 
tudes of  Central  Siberia,  has  not  been  determined. 

The  Increase  of  the  Wind  Velocity  with  Increase  of  Alti- 
tude is  very  rapid  for  the  first  hundred  or  two  hundred  feet ; 
but  above  that  it  is  slow,  and  very  variable  not  only  for 
the  yearly  averages,  but  also  for  the  different  months  of 
the  year. 

It  is  quite  probable  that  up  to  some  unknown  altitude 
the  wind  velocity  increases,  and  at  higher  altitudes  it  de- 
creases again.  The  increase  in  wind  velocity  with  altitude 
is  very  much  less  over  the  ocean  than  on  the  land,  for 
the  first  few  hundred  feet;  but  above  this  altitude  there  is 
probably  not  much  difference  between  the  wind  velocities 
over  a  land  and  those  over  a  water  surface.  The  move- 
ments of  clouds  show  that  wind  velocities  of  perhaps  200 
miles  per  hour  sometimes  occur  at  high  altitudes. 

Obstacles  to  Air  Motions.  —  Air  movement  does  not 
progress  unhindered,  for  the  moving  air  layers  near  the 
ground  rub  against  it,  and  those  up  above  rub  against 
other  air  layers  which  have  a  different  direction  of  motion, 
or  no  motion  at  all.  This  rubbing  is  called  friction,  and  it 
retards  the  air  currents. 

The  friction  of  one  current  of  air  on  another  is  exceed- 


112  ELEMENTARY    METEOROLOGY. 

ingly  slight ;  but  the  friction  of  the  air  against  the  earth  is 
very  great. 

The  main  hindrance  to  the  air  currents  acquiring  enor- 
mous velocities  is  the  continual  mixing  or  mingling  of  air 
layers  having  different  directions  of  motion,  and  especially 
the  breaking-up  of  the  air  into  vortices,  within  which  the  air 
layers  which  had  the  initial  velocities  become  so  increased 
in  numbers,  and  separated,  and  so  broken  up  into  strata 
which  twist  spirally  around  one  another,  that  the  friction, 
and  especially  the  interference  of  such  innumerable  sur- 
faces having  inequalities  of  motion,  becomes  very  great, 
and  the  motion  is  thus  equalized.  Mountain  ranges  may 
also  somewhat  retard  the  velocities  of  the  lower  air  currents. 

The  Decrease  of  Wind  Velocities  through  Friction  with 
the  Ground  is  well  shown  by  comparing  the  wind  velocities 
on  the  land,  where  the  friction  and  direct  obstacles  are 
greatest,  with  those  on  the  ocean,  where  they  are  least. 

It  is  found  by  observation  that  the  wind  velocities 
over  the  land  at  the  height  of  low  buildings  (40  or  50  feet 
above  ground)  are  about  25  %,  at  the  height  of  the  tallest 
buildings  (looto  150  feet  above  ground)  about  50%,  and 
on  the  well-exposed  seashore  about  75  %,  of  the  normal 
velocities  existing  on  the  open  ocean  (40  feet  above  the 
water)  for  the  same  geographical  locality. 

The  Variation  of  Lower  Wind  Velocity  with  the  Latitude 
is  not  easy  to  determine  in  amount,  on  account  of  the 
different  exposure  of  anemometers  ;  but  it  is  thought  that 
in  the  northern  hemisphere,  at  least,  there  is  an  increase 
in  wind  with  increase  of  latitude  up  to  latitude  50°- 60°, 
and  then  a  decrease  farther  poleward. 

The  discussion  of  a  few  observations  on  the  open  Atlantic  Ocean 
showed  an  increase,  in  the  average  wind  for  the  whole  year,  of  from 
about  8  miles  per  hour  in  latitude  22D  or  23°  north,  to  17  miles  per  hour 


WINDS.  113 

at  about  latitude  50°  north,  from  which  latitude  northward  to  58°  north 
there  was  again  a  slight  decrease  ;  but  how  this  decrease  continues 
onward  towards  the  pole  has  not  been  determined. 

The  general  region  of  maximum  wind  in  middle  and  higher  latitudes 
is  shown  for  the  Atlantic  Ocean  in  Figs.  36  and  37,  which  give  for 
January  and  July  the  wind  relations  of  the  Atlantic  Ocean  as  regards 
both  the  direction,  and  roughly  the  force,  of  the  wind.  The  heavier 
arrows  denote  the  stronger  wind,  and  the  double  arrows  the  strongest 
wind;  short  arrows  denote  variable,  and  long  arrows  steady  wind; 
circles  show  region  of  prevailing  calms.  These  show  in  a  general  way 
the  increase  in  wind  velocities  from  the  tropics  up  to  the  higher  middle 
latitudes. 

The  Periodic  Daily  Change  in  the  Wind's  Direction  is  rela- 
tively slightly  marked,  but  still  it  is  sufficient  to  have  been 
recognized.  It  stands  in  close  connection  with  the  daily 
variation  in  velocity,  since  the  vertical  air  currents  occur- 
ring about  midday  and  in  the  early  afternoon  communi- 
cate not  only  part  of  their  velocity,  but  also  their  direction, 
to  the  lower  air  currents.  The  air  currents  above  have  a 
tendency  to  move  or  deviate  to  the  right  of  the  lower  air 
currents,  and  this  motion  causes  in  the  northern  hemi- 
sphere, over  a  level  land  surface,  a  tendency  for  the  wind 
in  the  morning  to  turn  in  the  direction  of  the  hands  of  a 
watch  or  movement  of  the  sun,  and  towards  evening  to  turn 
in  the  opposite  direction ;  but  in  the  higher  atmosphere  (as 
on  high  mountain  peaks)  this  is  reversed.  In  the  southern 
hemisphere,  on  the  level  land  surface,  the  morning  turning 
is  in  the  direction  opposite  to  the  motion  of  the  hands  of 
a  watch,  and  towards  evening  with  it ;  and  in  the  upper 
atmosphere  this  is  reversed. 

At  the  equator  the  vertical  air  currents  do  not  cause  this 
diurnal  turning  of  the  wind's  direction. 

On  the  ocean  the  tendency  for  the  wind's  direction  to 
turn  as  described  is  very  slight  on  account  of  the  weak 

WALDO    METEOR. — 7 


w~5i  r     ^*  •>>O  i — m,  r~^^^  ir    '^S's'ji  >£  >VH  ~~* 

y/Pvv-~fT5><5/lo/ Pvl  -4  u-  L-r  -^^Kx  ^//k-^  c^-- -- 


Longitude   (i()         W,.st         10        I'min         :iO    Greenwich  0     Longitude   20    Kast 

FIG.  36. —WINDS  or  THE  ATLANTIC  OCEAN,  JANUARY  (DEUTSCHE  SEEWARTE). 


100  80 


^M±o 


80  Longitude  60        West       40       from        20  Greenwich  0   Longitude   20    East 


FIG.  37.  — WINDS  OF  THE  ATLANTIC  OCEAN,  JULY  (DEUTSCHE  SEEWARTE). 

(H5) 


ELEMENTARY   METEOROLOGY. 


vertical  currents  which  arise  over  the  relatively  cool  water 
in  the  daytime. 

The  local  diurnal  changes  in  the  wind's  direction  (known 
as  sea  and  land  breezes,  and  mountain  and  valley  winds)  are 
described  in  treating  of  the  atmospheric  motions  (p.  262). 

The  Periodic  Annual  Changes  in  the  Wind's  Direction  are 
due  to  the  seasonal  shifting  of  the  temperature  conditions 
which  have  already  been  mentioned. 

First,  there  is  the  change  in  the  general  circulation  of 
the  atmosphere,  due  to  the  shifting  of  the  region  of  greatest 
heat,  first  to  the  north,  and  then  to  the  south,  of  the  equator. 

Secondly,  there  is  an  inflow  of  the  lower  air  towards  the 
center  of  the  continents  in  summer,  and  an  outflow  from 
the  continents  in  winter.  These  are  called  the  monsoon 
winds ;  and  they  cause  reversals  of  the  direction  of  the 
general  flow  of  air  in  the  regions  affected. 

Thirdly,  there  is  a  seasonal  variation  in  the  general 
courses  or  tracks  of  barometric  minima  which  move  across 
the  earth's  surface,  and  this  changes  the  general  direction 
of  the  wind  in  the  region  lying  in  their  paths. 

In  the  middle  latitudes,  where  these  annual  changes  are  most 
marked,  the  direction  of  the  wind  is  a  most  important  factor  in 
determining  the  climate  of  the  region. 

The  resultant  wind  directions  at  selected  stations  are,  — 


Jan. 

Feb. 

March 

April 

May 

June 

St.  Paul  .  . 
Key  West  . 

S.73°W. 
N.  59°  E. 

s.  7o-  w. 

N.  66°  E. 

N.  62-  \V. 
N.  76°  E. 

N.  10°  W. 
N.  87°  E. 

S.  75°  E. 

N.  82°  E. 

S.  4°  E. 
S.  65°  E. 

July 

Aug. 

Sept. 

Oct. 

Nov. 

Dec. 

Year 

St.  Paul  .  .   . 
Key  West  .  . 

S.  31°  W. 
S.  71°  E. 

S.  4°  E. 
S.  76°  E. 

N.  88°  E. 

S.37°W. 

N.  60°  E. 

S.  *2°  W. 

N.54°E. 

S.79-W. 

N.53°E. 

N.  78°  E. 

WINDS. 

The  resultant  direction  of  the  horizontal  and  vertical  components 
of  the  wind  has  never  been  determined  with  any  great  accuracy, 
but  it  must  on  the  average  amount  to  only  a  few  degrees  from  the 
horizontal. 

Formation  of  Air  Waves.  —  It  is  seen  that  when  the 
wind  blows  over  a  water  surface  there  results  the  well- 
known  phenomenon  of  water  waves.  Similarly,  if  one  cur- 
rent of  air  moves  over  another  at  rest  or  of  a  different 
velocity  or  direction,  and  the  line  of  demarcation  between 
the  two  is  sharply  drawn,  there  will  be  a  favorable  con- 
dition for  the  formation  of  air  waves.  Such  conditions  do 
occur  very  frequently  in  our  atmosphere,  and  since  the 
effects  of  friction  between  two  air  currents  moving  one 
over  the  other  are  so  very  slight,  we  can  have  very  great 
differences  in  velocity  existing  between  the  two  within  a 
short  distance  of  the  boundary  between  them.  Air  waves 
have  a  magnitude  2,500  times  that  of  water  waves  pro- 
duced by  the  same  wind  conditions.  The  summits  or  crests 
of  air  waves  are  made  visible  by  some  of  the  detached 
clouds  high  up  in  the  sky ;  and  their  troughs  are  made 
evident  by  the  gusts  of  wind  felt  in  their  passage  near  the 
surface  of  the  ground. 

The  further  discussion  of  the  winds  will  be  taken  up  in  other  chap 
ters  on  the  subject  of  the  movements  of  the  atmosphere. 


CHAPTER  V. 
MOISTURE:  VAPOR,  CLOUD. 

Moisture  of  the  Air.  —  There  is  a  continually  repeated 
interchange  of  water  going  on  between  the  air  and  the 
surface  of  the  earth,  and  in  considering  the  circuit  through 
which  the  water  passes  in  these  changes  it  is  most  con- 
venient to  separate  it  into  three  sections ;  viz.,  — 

1.  The  moisture  as  it  exists  in  the  atmosphere  as  invis- 
ible vapor  and  as  cloud,  or  the  hygrometry  of  the  air. 

2.  The  rate  at  which  it  condenses  and  descends  to  the 
earth  as  precipitation,  and  the  amount  of  this  precipita- 
tion ;  or  the  rain,  hail,  and  snow  fall. 

3.  The  rate  at  which  the  moisture  is  taken  up  again 
into  the  air  from  the  earth's  surface,  or  the  evaporation. 

Moisture  exists,  either  as  a  gas  or  vapor,  in  an  invisible 
form ;  or  as  a  liquid  or  solid,  in  visible  form.  The  change 
from  one  form  to  another  depends  mainly  on  the  con- 
ditions of  temperature.  For  a  given  temperature,  only  a 
certain  maximum  amount  of  moisture  can  exist  as  a  vapor. 
When  this  temperature  is  reached  by  cooling,  saturation 
occurs,  and  any  further  cooling  condenses  the  moisture,  and 
renders  it  visible. 

Evaporation  is  that  process  by  which  a  liquid  is  con* 
verted  into  its  vapor ;  as,  for  instance,  the  conversion  of 
water  into  water  vapor.  Some  solids  also  give  off  vapor, 
ice  among  others.  The  molecules  of  both  the  liquid  and 

118 


MOISTURE:    VAPOR,  CLOUD.  1 19 

its  vapor  are  in  constant  motion.  The  average  velocities 
in  the  vapor  are  greater  than  in  the  liquid ;  but  the  largest 
velocities  in  the  liquid  may  be  greater  than  the  smallest  in 
the  vapor.  If  any  of  the  molecules  at  the  surface  of  the 
liquid  have  these  abnormally  large  velocities,  and  if  they 
are  moving  from  the  liquid,  they  will  escape  from  those 
forces  which  retain  in  the  liquid  the  molecules  of  lesser 
velocity,  and  will  fly  off  from  the  liquid  as  vapor.  The 
number  of  molecules  which  pass  from  the  liquid  to  vapor 
depends  mainly  on  the  temperature,  but  partly  also  on  the 
amount  of  vapor  already  existing  above  the  liquid. 

In  the  process  of  evaporation  there  is  a  disappearance  of  a  quantity 
of  heat,  and  a  depression  of  temperature  occurs.  This  heat  which  dis- 
appears is  called  the  latent  heat  of  vaporization.  Its  amount  varies 
according  to  the  temperature  at  which  evaporation  takes  place.  For  a 
further  account  of  this,  the  reader  must  consult  a  text-book  of  physics. 

Condensation.  —  If  the  molecules  of  vapor  have  their 
velocities  diminished,  by  cooling,  down  to  the  velocities  of 
the  liquid,  then  condensation  occurs,  and  the  vapor  is  con- 
verted into  a  liquid ;  or  the  molecules  of  vapor  striking 
the  liquid  may  become  entangled  among  the  molecules  of 
the  liquid,  and  thus  become  part  of  the  liquid.  The  num- 
ber of  molecules  which  pass  from  the  vapor  to  the  liquid 
depends  on  the  density  of  the  vapor  as  well  as  on  the 
temperature. 

Saturation.  —  If  the  temperature  of  the  vapor  and  liquid 
is  the  same,  evaporation  will  be  in  excess  of  condensation 
until  the  density  of  the  vapor  has  become  so  great  that 
as  many  molecules  are  condensed  as  are  evaporated,  then 
the  vapor  has  attained  its  maximum  density  (for  that  tem- 
perature), and  is  said  to  be  saturated.  Evaporation  and 
condensation  are  still  going  on,  however,  after  saturation ; 
but  they  just  balance  each  other. 


I2O  ELEMENTARY   METEOROLOGY. 

Conditions  of  the  Atmosphere  with  Regard  to  Moisture. — 
Four  conditions  of  the  atmosphere  with  respect  to  moisture 
are  recognized,  which  may  be  termed  stages,  since  they 
are  brought  about  by  a  progressive  succession  of  changes 
to  which  the  moist  air  is  subjected. 

There  is  first  the  dry  stage,  in  which  the  air  contains 
vapor;  but  it  is  not  saturated,  and  consequently  no  con- 
densation or  precipitation  takes  place.  The  amount  of 
unsaturated  or  superheated  vapor  in  the  dry  stage  for  the 
free  air  (during  its  changes)  may  be  assumed  to  remain 
constant;  that  is,  no  vapor  is  added  or  abstracted.  In  this 
dry  stage,  if  the  air  mixture  has  been  cooled,  then  by  the 
addition  of  heat,  or  by  adiabatic  compression,  the  original 
condition  can  be  resumed,  as  no  water  has  been  lost. 

The  rain  stage  is  reached  when  the  unsaturated  moist 
air  of  the  dry  stage  is  cooled  by  the  abstraction  of  heat,  or 
subjected  to  adiabatic  expansion  until  the  condition  of 
saturation  is  reached  and  passed,  and  water  drops  form. 

The  water  drops  fall  away  from  the  air  as  rain,  unless, 
by  means  of  a  strong  upward  wind,  they  are'  carried 
along  with  the  air  in  which  the  condensation  occurred. 
The  absolute  amount  of  moisture  in  the  air  is  decreased  by 
the  amount  which  falls  away  as  rain,  and  the  remaining 
vapor  is  in  a  saturated  condition. 

In  the  rain  stage  the  changes  which  occur  are  seldom  reversible, 
that  is,  capable  of  returning  the  air  to  its  original  condition,  as  has  been 
mentioned  for  the  dry  stage,  because  usually  some  of  the  moisture  be- 
comes lost  as  raindrops.  In  the  rain  stage  there  are  two  extreme  con- 
ditions which  may  exist :  either  all  of  the  water  drops  which  have  been 
rormed  may  fall  away  as  rain,  or  the  water  drops  may  be  carried  along 
with  the  air  from  which  they  were  condensed,  by  means  of  the  support 
offered  by  sufficiently  powerful  vertical  air  currents.  In  the  former 
case  the  temperature  will  fall  more  rapidly  than  in  the  latter  as  the 
change  goes  on. 


MOISTURE:   VAPOR,  CLOUD.  121 

The  hail  stage  is  reached  when  the  temperature  of  the 
cooling  moist  air  reaches  the  freezing  point  of  water. 
The  air  mixture  in  this  stage  is  made  up  of  dry  air,  satu- 
rated vapor,  any  water  drops  which  are  present,  and  frozen 
water  drops  which  exist  as  ice  or  hail.  When  the  tempera- 
ture is  just  at  the  freezing  point,  water  drops  and  hail  pel- 
lets may  exist  side  by  side. 

At  the  beginning  of  the  hail  stage  we  had  the  rain 
stage,  in  which  the  air  mixture  contained  water  in  the  form 
of  raindrops  and  saturated  vapor.  The  amount  of  hail 
will  depend  on  the  amount  of  these  water  drops  when  the 
hail  stage  was  entered  upon,  and  also  on  the  amount  of 
change  in  the  volume  of  the  mixture  by  expansion.  It 
must  be  carefully  noted  that  the  hail  stage  cannot  be 
entered  upon  until  there  is  an  appreciable  amount  of 
water  present  as  raindrops. 

The  snow  stage  is  reached  by  the  gradual  cooling  of  the 
ascending  air  to  a  temperature  below  freezing  of  water, 
but  without  the  formation  of  raindrops ;  or  when  the  pre- 
cipitation of  rain  and  hail  is  so  slight  that  the  latter  stage, 
at  least,  is  practically  skipped  over.  In  the  snow  stage 
the  air  mixture  consists  of  dry  air,  saturated  vapor,  and 
snowflakes,  and  the  mass  is  still  less  than  for  the  hail 
stage. 

Since  the  amount  of  vapor  which  may  be  present  depends  on  the 
temperature,  then,  the  lower  the  temperature,  the  less  the  possible  snow- 
fall ;  which  accounts  for  the  fact  of  comparatively  light  snowfalls  in 
extremely  cold  weather. 

By  a  reversion  of  the  process  by  which  these  stages 
are  reached  —  that  is,  by  a  direct  warming,  or  by  adiabatic 
compression  of  the  air  mixture — it  is  possible  to  carry  it 
back  into  the  dry  stage  again ;  but  the  amount  of  vapor 


122  ELEMENTARY   METEOROLOGY. 

will  be  less  than  that  it  started  with,  by  the  amount  of 
moisture  which  has  fallen  away  in  the  form  of  rain,  hail, 
or  snow. 

ATMOSPHERIC  MOISTURE  AS  VAPOR. 

The  Amount  of  Moisture  in  the  air  existing  as  a  vapor 
may  vary  from  nothing  up  to  saturation.  These  two 
extreme  conditions  serve  as  terminal  points  for  a  scale  of 
measurement  of  the  degree  of  moisture  in  the  air.  The 
temperature  is  the  most  important  factor  in  determining 
the  amount  of  moisture,  and  in  general  the  possible  maxi- 
mum amount  of  moisture  diminishes  with  decrease  of  tem- 
perature ;  or,  the  colder  the  air,  the  less  the  possible  amount 
of  moisture. 

There  are  three  terms  constantly  used  in  speaking  of 
atmospheric  moisture :  absolute  humidity,  relative  humid- 
ity, and  the  dew-point. 

Atmospheric  Humidity.  —  The  humidity  of  the  air  is  the 
amount  of  moisture  which  it  contains ;  that  is,  the  amount 
which  is  mixed  with  the  dry  air  to  form  the  atmospheric 
air. 

The  Absolute  Humidity  is  the  weight  of  this  moisture. 
If  we  consider  it  in  its  watery  condition,  it  is  then  ex- 
pressed in  grains  per  cubic  foot  of  air.  If  we  consider 
it  in  its  invisible  gaseous  condition,  then  we  measure  the 
pressure  or  tension  of  the  vapor,  and  it  is  then  expressed 
in  fractions  of  an  inch  of  barometric  pressure. 

The  Relative  Humidity  is  the  relation  of  the  amount  of 
moisture  present  to  the  amount  necessary  for  saturation 
under  the  existing  condition,  and  it  is  expressed  in  per- 
centage of  the  latter.  When  it  is  said  that  the  relative , 
humidity  is  50  %,  this  means  that  half  as  much  moisture 
is  present  as  would  be  necessary  for  the  saturation  of  the 


MOISTURE;   VAPOR,  CLOUD.  123 

vapor  under  the  existing  conditions  of  temperature  and 
barometric  pressure. 

The  Dew-point.  —  Since  the  amount  of  vapor  which  can 
exist  in  the  air  depends  chiefly  on  the  temperature  of  the 
air,  then,  if  we  make  any  moist  air  colder,  we  shall  increase 
its  relative  humidity ;  and  if  it  is  cooled  far  enough,  the 
relative  humidity  will  become  100  %,  and  saturation  will 
OCCUH 

The  dew-point  is  that  temperature  of  the  air  at  which 
its  invisible  moisture  begins  to  condense  into  visible  water 
drops. 

The  following  table  shows  the  weight,  in  grains  Troy,  of  saturated 
vapor  per  cubic  foot  at  the  annexed  dew-point  temperatures  :  — 


Dew-point,  F°   .  .  .   . 

0° 
1.7 

40° 
2.Q 

60° 

5,8 

80° 

II.O 

100° 
IQ.8 

If  a  little  ice  is  put  into  a  tin  cup  of  water,  and  the  water  is  stirred  so 
that  all  of  the  water  may  cool  at  the  same  time,  and  if  the  bulb  of  a 
thermometer  is  put  into  the  water  and  the  temperature  is  noted  when 
the  moisture  first  begins  to  be  deposited  on  the  outside  of  the  cup  (like 
the  "sweating"  of  a  pitcher  of  cold  water  on  a  warm  day),  then  the 
thermometer  reading  gives  the  dew-point  of  the  air  for  its  existing 
conditions.  It  is  a  little  difficult  to  detect  the  exact  instant  at 
which  the  condensation  of  the  moisture  on  the  outside  of  the  cup 
begins,  and  so  it  is  usual  to  note  as  accurately  as  possible  the  tempera- 
ture when  this  condensation  begins,  and  then  to  warm  the  water  slightly 
and  note  its  temperature  when  the  condensed  moisture  disappears  ;  and 
the  average  of  the  two  temperatures  will  give  quite  closely  the  desired 
dew-point. 

Methods  of  Measuring  Atmospheric  Moisture.  —  There  are  several 
methods  of  determining  the  amount  of  moisture  in  the  atmosphere, 
but  those  in  chief  use  by  meteorologists  are:  (i)  by  determining  the 
temperature  of  evaporation  (by  using  the  psychrometer)  ;  (2)  by 
observing  the  proportional  saturation  of  certain  animal  and  vegetable 
substances  (by  using  the  hair  hygrometer) ;  (3)  by  observing  the 
various  aspects  of  the  sky. 


124 


ELEMENTARY   METEOROLOGY. 


Determination  of  Humidity  by  the  Temperature  of  Evapo- 
ration.—  The  psyclirometer  is  used  in  this  method.  This 
instrument  consists  of  two  similar  mercurial  thermometers 
placed  side  by  side ;  the  one  being  in  its  unaltered  condi- 
tion, and  the  other  having  a  wet  thin  muslin  covering  for 

the    bulb    which    contains    the 

mercury.  The  muslin  cloth  is 
usually  made  wet  by  connec- 
tion with  a  wicking  dipped  into 
water.  The  evaporation  of  the 
water  from  the  cloth  cools  the 
thermometer  bulb  to  the  tem- 
perature of  evaporation;  and 
the  drier  the  air,  the  lower  will 
the  temperature  of  the  wet  bulb 
sink  below  that  of  the  dry  bulb. 
A  psychrometer  is  shown  in 
Fig.  38.  More  accurate  results 
than  for  still  air  are  obtained  by 
making  an  air  current  pass  over 
the  thermometer  bulb,  which 
can  be  done  most  easily  by 
whirling  the  thermometers. 
The  Hair  Hygrometer  (Fig.  39) 

FIG.  38.  — PSYCHROMETER;  WET  AND     js  sometimes   USed   for  determin- 
DRY  BULB  THERMOMETERS.  .  ...  ...  ... 

ing  the  relative  humidity  of  the 

atmospheric  air.  This  instrument  is  based  on  the  fact  that 
hair  expands  in  length  with  increase  of  moisture,  and  con- 
tracts with  decrease  of  moisture.  The  amount  of  this  ex- 
pansion or  contraction  is  measured  on  a  scale  which  shows 
the  percentage  of  total  expansion  for  saturation. 

Actual  Observation  of  Atmospheric  Humidity,  —  The 
atmospheric  relative  humidity  most  directly  affects  our 


MOISTURE:   VAPOR,  CLOUD. 


125 


sensibilities,  and  it  is  generally  expressed  in  per  cent  of 
saturation.  For  many  questions  of  physical  investigation 
concerning  the  atmosphere,  however,  it  is  necessary  to 
know  the  absolute  amount  of  water  in  the  atmospheric  air, 
expressed  either  in  vapor  pressure  or  in  weight  of  water. 

The  Daily  March  of  the  Relative  Humidity  is  very 
pronounced.  In  general  it  decreases  with  the  diurnal 
increase  of  temperature.  There  is  an  early  morning 
maximum  and  an  afternoon 
minimum.  During  the  day- 
time, when  the  temperature 
increase  is  rapid,  just  before 
and  about  noon,  the  relative 
humidity  falls  very  rapidly, 
but  during  the  night  it  does 
not  vary  by  a  great  amount. 
Where  there  is  a  large  daily 
amplitude  for  the  oscillation 
of  temperature,  there  will 
also  be  found  a  great  ampli- 
tude for  the  relative  humid- 
ity ;  and  where  the  amplitude 
of  the  temperature  is  small, 
that  of  the  relative  humidity  will  likewise  be  small.  The 
diurnal  amplitude  of  the  relative  humidity  is  small  near 
the  ocean  coasts,  but  increases  towards  the  interior  of  the 
continent. 


FIG.  39.  — HAIR  HYGROMETER. 


On  the  northwest  coast  of  Europe  the  amplitude  is  7%  in  winter 
(December),  and  17%  in  summer  (August)  ;  while  at  Nukuss,  in  cen- 
tral Asia,  the  amplitude  is  26%  in  winter  (December),  and  over  50% 
in  summer  (August). 

The  relative  humidity  has  the  greatest  amplitude  on  clear  days,  and 
the  least  on  cloudy  days.  The  amplitude  is  several  times  as  great  in 


126 


ELEMENTARY   METEOROLOGY. 


clear  weather  as  in  rainy  weather.  There  is  a  tendency  for  the  rela- 
tive humidity  to  increase  on  the  average  for  the  24  hours  of  cloudy 
days,  and  to  decrease  during  clear  days. 

The  Annual  March  of  the  Relative  Humidity  is  somewhat 
irregular.  The  time  of  maximum  is  in  midwinter,  and  of 
minimum  in  early  summer.  The  annual  amplitude  is  least 
at  the  coast  stations,  and  greatest  in  the  interior  of  con- 
tinents. The  amplitude  for  the  extreme  months  on  the 
coast  of  the  ocean  amounts  to  perhaps  5  %,  and  in  the 
interior  of  the  continent  to  over  30  %. 

The  average  annual  relative  humidity  is  greatest  for  the 
marine  climate,  and  decreases  towards  the  interior  of  the 
continents.  Up  to  a  certain  altitude  (that  of  the  clouds), 
the  annual  amount  increases  with  the  elevation,  but  be- 
yond that  point  it  decreases.  But  it  varies  greatly  with 
altitude,  since  the  temperature  and  amount  of  vapor,  upon 
which  relative  humidity  depends,  vary  so  much  with  alti- 
tude. 

Local  influences,  such  as  mountains,  the  direction  of  the  winds  with 
regard  to  the  neighboring  drier  or  moister  regions,  and  other  causes, 
affect  the  relative  humidity  of  a  place. 

The  annual  march  of  the  relative  humidity  at  a  few  stations  is  shown 
in  percentage  by  the  following  average  monthly  relative  humidities :  — 


J- 

F. 

M. 

A. 

M. 

J- 

J- 

A. 

S. 

O. 

N. 

D. 

Year 

Key  West,  Fla.    .     . 

80 

77 

70 

69 

7i 

7i 

70 

72 

75 

76 

78 

79 

74 

St.  Paul,  Minn.   .     . 

72 

71 

69 

60 

60 

68 

70 

72 

7» 

69 

72 

74 

69 

Salt  Lake  City,  Utah 

60 

57 

58 

45 

40 

32 

30 

31 

3i 

42 

52 

60 

44 

The  Geographical  Distribution  of  the  Relative  Humidity 

is  not  easily  shown  on  charts,  on  account  of  its  variability 
with  altitude.     On  the  open  ocean,   however,   it  is  quite 


MOISTURE:    VAPOR,  CLOUD.  1 2? 

regular,  and  on  the  average  varies  between  82%  at  the 
equator  and  92%  in  high  latitudes.  On  the  continents, 
owing  to  the  high  temperatures,  there  is  usually  a  small 
relative  humidity  in  summer,  and  a  large  one  in  winter ; 
but  for  the  absolute  humidity  we  have  just  the  reverse  of 
this. 

The  Daily  March  of  the  Absolute  Humidity  varies  greatly 
for  different  places.  There  are,  however,  two  pronounced 
types,  —  the  maritime  and  the  continental. 

The  maritime  type  has  a  maximum  in  the  warmest  part 
of  the  day,  with  an  early  morning  minimum.  The  conti- 
nental type  has  two  maxima,  one  just  before  noon,  and  the 
other  late  in  the  afternoon,  with  a  secondary  minimum 
between  ;  the  primary  or  lowest  minimum  occurring  in  the 

early  morning. 

i 

On  the  mountains  the  amplitude  is  less  than  on  the  plains.  The 
secondary  minimum  in  the  continental  type  is  due  to  the  ascending  air 
currents  which  occur  towards  the  hottest  part  of  the  day ;  the  surface 
air  being  carried  upward  more  rapidly  than  the  moisture  is  supplied  to 
it  by  the  evaporation  which  takes  place  from  the  earth's  surface.  Over 
the  water  surface  these  ascending  currents  are  not  so  strong  as  over 
the  land  (especially  on  dry  plains),  and  therefore  the  air  is  not  so  likely 
to  rise  upward  faster  than  the  lower  layers  are  supplied  with  moisture 
by  evaporation. 

The  absolute  humidity  has  the  greatest  amplitude  on  clear  days,  and 
the  least  on  cloudy  days,  when  precipitation  also  occurs.  There  is  a 
tendency  for  the  absolute  humidity  to  decrease  during  the  24  hours  of 
clear  days,  and  to  increase  during  cloudy  days. 

The  Annual  March  of  the  Absolute  Humidity  is  very 
similar  in  character  to  that  of  the  temperature.  The 
minimum  is  in  the  winter  months,  and  the  maximum 
in  the  midsummer  months.  The  amplitude  for  the  year 
is  least  on  the  coasts,  and  greatest  in  the  interior  of  con- 


128 


ELEMENTARY   METEOROLOGY. 


tinents ;  and  it  is  less  in  the  tropics  than  in  the  temperate 
zones. 

The  Average  Amount  of  Vapor  decreases  with  the  alti- 
tude above  sea  level,  and  it  decreases  more  rapidly  than 
the  decrease  of  air  pressure  (and  density)  with  altitude. 

The  relative  decrease  of  the-  amount  of  water  vapor  with  the  altitude, 
and  also  the  decrease  of  the  air  density,  are  shown  by  the  following 
table,  in  which  the  air  density  and  water  vapor  at  the  earth's  surface 
are  each  placed  at  1 .00 :  — 


ALTITUDE. 

WATER  VAPOR. 

AIR  DENSITY. 

Feet. 

O 

1.  00 

1.  00 

13,000  + 

.24 

.61 

30,000- 

.04 

•32 

It  is  seen  how  much  more  rapidly  the  water  vapor  decreases  than 
the  air  density.  Half  of  the  water  vapor  lies  below  the  altitude  of 
about  6,500  feet,  while  half  of  the  air  lies  below  the  altitude  of  about 
18,000  feet.  The  low  altitude  of  the  greater  portion  of  the  moisture  in 
the  atmosphere  accounts  in  a  great  measure  for  the  powerful  influence 
which  even  low  mountains  exert  in  the  distribution  of  this  moisture 
over  the  earth's  surface. 

The  Geographical  Distribution  of  the  Water  Vapor  over  the 
different  regions  of  the  earth  follows  quite  closely  that  of 
the  temperature ;  in  general  the  greater  the  temperature, 
the  greater  the  amount  of  water  vapor.  With  increase  of 
latitude  the  amount  of  vapor  decreases.  In  January  the 
region  of  greatest  amount  of  vapor  is  the  region  extending 
from  the  equator  to  about  latitude  20°  south. 

In  this  region  the  vapor  pressure  is  over  0.8  of  an  inch,  but  in 
equatorial  Africa  it  reaches  i  inch,  and  on  the  northern  coast  of  Aus- 
tralia 0.95  of  an  inch.  In  western  Europe  it  varies  from  0.2  to  0.4  of 


MOISTURE:    VAPOR,  CLOUD.  1 29 

an  inch,  in  eastern  Europe  from  o.i  to  0.2  of  an  inch,  and  in  the  cold 
regions  of  northeastern  Asia  (and  likewise  in  north-central  North  Amer- 
ica) it  decreases  to  0.05  of  an  inch  or  lower. 

In  July  the  region  of  highest  vapor  pressure  lies  to  the 
north  of  the  equator,  where  in  India  it  reaches  I  inch. 

In  the  extreme  northern  part  of  the  northern  hemisphere  the  vapor 
pressure  is  reduced  to  0.2  of  an  inch  or  less ;  while  in  the  southern 
hemisphere  the  pressure  of  0.2  of  an  inch  is  reached  even  at  latitude 
40°  south. 

It  is  thus  seen  that  there  is  a  movement  of  the  maximum 
region  backwards  and  forwards  across  the  equator  from 
one  hemisphere  to  the  other,  following  the  sun. 

ATMOSPHERIC  MOISTURE  AS  CLOUD  AND  FOG. 

Fog  and  Cloud  Formation. — When  air  is  cooled  just 
below  the  dew-point,  then  fog  or  cloud  occurs,  and,  as  may 
be  seen  by  observing  the  sky,  in  a  great  variety  of  forms. 
While  in  a  general  way  the  process  of  cloud  formation  is 
understood,  yet  the  exact  manner  of  the  building  of 
clouds  of  the  various  shapes  and  appearances  which  occur 
has  not  been  entirely  studied  out.  Clouds  are  at  present 
divided  into  classes,  according  to  the  forms  which  they  pre- 
sent to  the  eye,  and  not  according  to  their  processes  of 
formation. 

Composition  of  Clouds.  —  Since  the  clouds  contain  more 
particles  of  dust  than  the  surrounding  clear  air,  it  is  con- 
cluded that  the  air  within  clouds  has  been  drawn  up  from 
close  to  the  earth's  surface,  where  these  dust  particles 
abound.  The  dust  particles  within  clouds  usually  number 
only  a  few  thousand  per  cubic  centimeter;  but  the  water 
particles  may  number  as  high  as  50,000  in  the  same  space. 

WALDO    METEOR.  —  8 


I3O  ELEMENTARY   METEOROLOGY. 

The  density  of  a  cloud  depends  directly  on  the  number 
of  water  particles  present. 

When  cloud  forms,  precipitation  begins  at  once ;  but 
the  water  particles  are  very  small,  and  evaporation  takes 
place  when  they  fall  into  drier  air  below.  The  distance 
which  they  fall  depends  on  their  size  and  the  dryness  of 
the  air  beneath.  The  denser  the  cloud,  the  larger  are 
the  raindrops,  and  the  faster  they  fall. 

Nomenclature  of  Cloud  Forms.  —  There  are  four  distinct 
classes  of  clouds,  according  to  their  forms,  —  cirrus,  stratiis, 
cumulus,  and  nimbus. 

Cirrus  clouds  are  those  which  are  seen  in  striated  forms 
grouped  high  up  in  the  sky. 

Stratus  clouds  are  those  which  present  a  stratified  or 
bank-like  form.  They  may  be  high  or  low  lying  clouds. 

Cumulus  clouds  are  more  or  less  isolated,  have  rounded 
tops,  and  are  found  at  but  moderate  altitudes  above  the 
ground. 

Nimbus  clouds  are  those  from  which  rain  or  snow 
descends. 

Combinations  of  these  forms  add  several  varieties  with 
recognized  names.  The  principal  ones  are  as  follows :  cir- 
rus, cirro-stratus,  alto-stratus  (strato-cirrus),  stratus,  cirro- 
cumulus,  alto-cumulus  (cumulo-cirrus),  strato-cumulus, 
cumulus,  cumulo-nimbus,  nimbus.  These  various  forms 
are  represented  in  the  accompanying  illustration  (Fig.  40). 

i.  Cirrus  clouds  (No.  i)  are  feathery  in  form  and  delicately  fibered, 
usually  of  a  white  color,  and  well  outlined  against  the  sky  background 
They  lie  arranged  in  a  variety  of  fantastic  forms.  Nearly  parallel  groups 
of  these  clouds  are  sometimes  seen  stretching  across  the  heavens  in 
converging,  meridian-like  bands.  Such  bands  as  these  are  also  some- 
times formed  of  cirro-stratus  and  cirro-cumulus  clouds.  The  cirrus 
clouds  have  perhaps  an  average  altitude  of  between  5  and  6  miles. 


MOISTURE:    VAPOR,  CLOUD.  .     131 

2.  Cirro-stratus  clouds  (No.  2),  consisting  of  fine,  white,  veil-like 
clouds,  and  the  alto-stratus  clouds  (No.  5),  thick,  veil-like  clouds  of 
grayish  or  bluish  color,  look  in  some  respects  quite  alike  in  form,  but 
their  altitudes  are  very  different ;  the  altitude  of  the  cirro-stratus  being 
about  5>£  miles  on  the  average,  and  that  of  the  alto-stratus  being  only 
about  half  as  great,  perhaps  about  3  miles.     The  cirro-stratus  clouds 
usually  precede  bad  weather,  and  they  gradually  give  place  to  alto- 
stratus  clouds.     The  cirro-stratus  are  the  clouds  which  give  rise  to  the 
phenomenon  of  rings  around  the  sun  and  moon.     The  cirro-stratus 
formation  is  sometimes  very  widely  diffused  over  the  heavens,  and  then 
becomes  a  cirrus  vapor ;  but  sometimes  a  distinct  but  intricate-fibered 
structure  is  visible,  which  may  be  called  cirrus-felt. 

3.  Cirro-cumulus  clouds  (No.  3)  are  very  small  white,  and  alto- 
cumulus  (No.  4)  large  whitish-gray,  balls  or  fleecy  clumps  grouped  in 
herds.     Sometimes  these  fleecy  clumps  are  arranged  in  rows,  extending 
in  one  or  two   directions.     The  cirro-cumulus  lie  much  the  higher, 
being  at  an  altitude  of  3^  to  4^  miles,  while  the  alto-cumulus  are 
only  about  2}^  miles  up. 

4.  Strato-cumulus  (No.  6)  and  nimbus  (No.  7)  clouds  belong  to  the 
lower  air  layers,  and  form  clumps  or  layers  at  an  altitude  of  from  ^ 
of  a  mile  to  iX  miles  above  the  ground.     Strato-cumulus  clouds  occur 
in  dry  weather,  and  appear  very  frequently  in  winter,  when  they  more 
or  less  cover  the  heavens,  but  with  patches  of  blue  sky  between  the 
clouds.      The  nimbus  is  the  cloud    of  continued  rain  or  snow.      It 
has  ragged  edges,  and  above  are  always  to  be  seen  the  alto-stratus 
clouds ;  so  that  between  the  nimbus  clouds  a  gray  cover  or  background 
is  visible,  and  not  the  blue  sky  such  as  is  seen  above  the  strato-curr.ulus. 

5.  Cumulus  clouds  (No.  8)  are  the  thick,  dense  clouds  with  rounded, 
festoon-like  tops  and  horizontal  bases.     The  top  usually  comes  to  a 
point  or  peak  higher  than  the  rest  of  the  cloud.     These  clouds  are 
built  up  by  ascending  currents  within  ;  and  when  these  air  currents  cease, 
the  clouds  disappear  gradually.     Such  clouds  are  characteristic  of  the 
summer  sky,  especially  over  the  land.     Sometimes  over  tropical  islands 
the  vertical  air  currents  which  arise  cause  more  or  less  permanent  cumu- 
lus clouds  to  overhang  the  islands. 

Cumulo-nimbus  clouds  (No.  9)  are  the  thunder  and  shower  clouds 
which  roll  up  in  such  an  imposing  manner,  and  present  a  majestic  ap- 
pearance of  mountain-like  character.  The  tops  are  of  a  light,  fluffy  ap- 
pearance, while  the  bases  are  of  the  dense  nimbus  character,  from  whose 


(132) 


FIG.  40. — PRINCIPAL  F 


s  OF  CLOUDS  (AFTER  KOPPEN). 


(133) 


134  ELEMENTARY   METEOROLOGY. 

center  showers  of  local  rains  and  hail  descend.  The  upper  edge  is 
festooned  like  the  cumulus,  and  towering  cumulus  peaks  are  formed. 
The  edges  are  sometimes  bordered  by  small,  cirrus-like  clouds,  or  these 
last  are  detached  from  the  main  cloud. 

6.  Stratus  clouds  (No.  10)  consist  of  an  elevated  fog.  Fog  which 
lies  at  the  ground  is  designated  simply  fog;  but  when  it  is  at  an  alti- 
tude of,  say,  a  thousand  or  more  feet  above  the  ground,  it  is  called 
stratus  cloud.  Such  is  dry-weather  fog. 

The  broken,  tattered  clouds  ^which  appear  at  low  altitudes  in  wei 
weather  are  called  fracto-nimbus •.  The  tattered  clouds  which  occur 
with  the  true  cumulus  clouds  are  called  fracto-cumulus . 

Processes  of  Cloud  Formation.  — The  following  processes 
are  those  by  which  fog  and  cloud  are  produced  :  — 

1.  Direct  cooling  of  the  moist  air  through  contact  with 
colder  bodies  or  through  loss  of  heat  by  radiation. 

2.  Adiabatic  cooling  of  ascending  moist  air. 

3.  Mixture  of  moist  air  of  different  temperatures  and 
humidities. 

These  are  mentioned  in  the  order  of  their  effectiveness 
for  the  production  of  cloud. 

Dissipation  of  Clouds.  —  Conversely,  fog  or  cloud  may 
be  dissipated  in  the  following  manner  :  — 

1.  Direct  warming  of  the  clouded  air  through  radiation 
or  contact  with  warm  bodies. 

2.  Adiabatic  heating  of  descending  clouded  air. 

3.  Mixture  of  the  clouded  air  with  other  air  masses  of 
proper  temperature  and  humidity. 

Ground  Fog.  — The  condensation  produced  by  direct  con- 
tact with  a  cold  body  and  by  loss  of  heat  through  outward 
radiation  is  that  of  ground  fog,  which  extends  upwards  to  a 
moderate  height  above  the  ground.  The  outward  radia- 
tion from  the  ground  on  clear  nights  cools  the  ground  very 
rapidly  ;  and,  as  soon  as  the  dew-point  is  passed,  condensa- 
tion begins  in  the  lowest  air  layers,  and  fog  forms.  The 


MOISTURE:    VAPOR,  CLOUD.  135 

upper  surface  of  this  fog  radiates  heat,  and  cools  the  air 
layers  just  above  it,  so  that  condensation  ensues  in  these 
layers.  Thus  the  process  of  fog  building  proceeds,  and 
the  fog  layer  becomes  thicker  and  thicker.  When  the 
solar  radiation  begins  to  make  itself  felt,  the  reverse  takes 
place ;  and  the  upper  layers  are  dissipated  first  by  the 
warming  of  their  upper  surface ;  then  the  next  layer  is 
dissipated ;  and  so  on  until  the  ground  is  reached,  when 
it  too  becomes  warm,  condensation  ceases,  and  the  fog 
entirely  disappears. 

The  reason  that  no  excessive  precipitation  occurs  in  this  process  is 
that  the  formation  of  the  fog  cloud  above  the  ground  prevents  the  fur- 
ther excessive  cooling  of  the  latter  by  continued  radiation.  The  fog 
growth  on  the  upper  limit  of  elevated  clouds  may  also  occur  in  the 
manner  just  described  ;  but,  in  order  to  produce  condensation  by  direct 
radiation  in  the  upper  air  layers,  there  must  exist  a  cloudiness  formed 
by  some  other  process,  or  by  means  of  a  framework  of  such  impurities 
as  smoke  particles. 

The  Formation  of  Clouds  through  Adiabatic  Expansion, 

and  their  dissipation  through  compression,  occur  where 
there  exist  ascending  and  descending  air  currents.  The 
huge  summer  clouds  with  rounded  tops  and  horizontal 
bases,  the  so-called  thunderclouds,  and  the  usual  rain 
clouds,  are  formed  by  this  process,  when  ascending  air 
currents  are  present,  and  the  air  thus  expands  and  cools; 
and  their  dissipation  occurs  for  descending  movements, 
whereby  the  air  is  compressed  again  and  made  warmer. 

The  Formation  of  Cloud  by  Mixture  of  Air  of  Different 
Temperatures  and  Humidities  is  a  much  more  complex 
matter  than  that  just  described ;  and  for  cloud  formation 
it  is  of  great  importance,  but  is  comparatively  unimportant 
for  causing  the  precipitation  of  moisture  to  the  ground. 

Condensation  takes  place  more  rapidly  when  a  current 


136  ELEMENTARY   METEOROLOGY. 

of  cool  moist  air  penetrates  a  large  mass  of  warm  moist 
air,  than  when  a  current  of  warm  moist  air  enters  a  mass 
of  cool  air. 

Condensation  does  not  always  proceed  gradually,  and  it  may  not 
occur  until  the  final  stages  of  cooling  by  mixing  are  reached,  when  it 
will  take  place  all  at  once.  Likewise  in  the  reverse  process  the  dissipa- 
tion of  the  cloud  may  be  retarded  until  the  condition  is  such  that  the 
whole  of  the  cloud  will  disappear  quite  suddenly .  In  the  case  of  the 
breath  leaving  the  nostrils  and  penetrating  the  cold  air,  the  condensa- 
tion does  not  take  place  until  actual  mixture  is  effected ;  and  when  this 
mixing  process  distributes  the  breath  further  through  a  larger  mass  of 
the  drier  colder  air,  the  fog  is  dissipated.  It  has  been  shown,  that,  in 
the  mixing  of  saturated  cool  air  with  larger  quantities  of  saturated 
warm  air,  the  warming  of  the  former  takes  place  at  first  rapidly,  and 
then  more  gradually  ;  but  when  the  cold  air  is  greatest  in  quantity,  then 
the  cooling  of  the  warm  air  becomes  more  rapid  as  the  process  con- 
tinues. The  maximum  amount  of  condensation  will  occur  when  the 
amount  of  cool  air  is  in  excess. 

By  this  process  of  mixing,  the  following  kinds  of  clouds  and  fog 
are  produced :  — 

1 .  The  fog  which  arises  over  relatively  moist  warm  surfaces,  where 
cooler  air  is  also  present.     An  example  of  this  is  the  surface  ocean 
fog  which  so  frequently  arises  during  the  cold  season. 

2.  The  clouds  which  are  formed  at  the  common  boundary  of  two  air 
currents  (the  air  in  which  may  differ  as  regards  both  temperature  and 
moisture)  moving  with  different  velocities,  by  which  means  a  regular 
succession  of  clouds  is  formed  as  a  consequence  of  a   wave   motion 
which  may  exist ;  and  so  great  is  the  amplitude  of  these  waves,  that  the 
adiabatic  condensation  arising  during  the  upward  surges  must  make 
itself  visible. 

3.  The  layers  of  stratus  clouds  which  are  formed  at  the  juncture 
surface  of  two  such  air  currents  having  different  velocities,  and  which 
have    at    first    a    disconnected   form,    but   which   afterwards   become 
joined. 

4.  The   banner-like    clouds   which    form    (and   dissolve)   on   some 
mountain  peaks,  and  in  mountain  passes,  when  the  contour  is  such  that 
warm  or  cold  masses  of  air  are  penetrated  by  currents  of  air  having 
other  temperatures. 


• 
MOISTURE:   VAPOR,  CLOUD.  137 

5.  The  loose  and  tattered  clouds  which  are  so  noticeable  in  strong 
winds,  and  especially  in  thunderstorms,  and  which  are  constantly 
undergoing  changes  of  form. 

It  is  very  probable  that  fogs  and  clouds  of  various  forms  are  also 
produced  by  combinations  of  any  two  or  all  three  of  the  causes  of 
condensation  ;  but  we  do  not  as  yet  know  their  method  of  forming  well 
enough  to  specifically  point  them  out.  It  may  also  be  remarked,  that, 
without  a  knowledge  of  the  mode  of  formation  of  clouds,  it  is  a  very 
easy  matter  to  misinterpret  their  movements  as  we  observe  them. 

Amount  of  Cloud.  —  The  degree  of  cloudiness  is  es- 
timated on  a  scale  of  from  o  when  there  are  no  clouds 
visible,  to  10  for  the  whole  sky  overcast  with  clouds.  It  is 
therefore  merely  estimated  how  many  tenths  of  the  sky 
are  covered  with  cloud.  Almost  all  of  such  observations 
are  made  by  the  unaided  eye.  These  estimations  are  re- 
duced to  percentage  of  the  total  visible  sky  by  multiplying 
them  by  10. 

While  there  has  been  some  slight  attempt  at  estimating  the  amount 
of  the  various  kinds  of  clouds,  yet  in  nearly  all  cases  the  degree  of 
cloudiness  is  estimated  irrespective  of  the  kind,  density,  or  height  of 
the  clouds.  Thus,  if  the  degree  of  cloudiness  is  recorded  as  7  (on 
a  scale  of  o  to  10),  and  cirrus  and  stratus  clouds  are  present,  no 
attempt  is  made  to  state  what  proportion  of  this  7  refers  to  the  cirrus 
and  what  to  the  stratus  clouds. 

The  Daily  March  of  Degree  of  Cloudiness  is  somewhat 
difficult  to  determine,  since  the  amplitudes  are  small,  and 
vary  only  about  10  %  of  the  whole  surface  of  the  sky.  In 
the  morning  hours  fog  and  stratus  clouds  are  the  most 
frequent;  but  towards  midday,  and  in  the  afternoon,  the 
cumulus  are  in  excess.  In  general  there  is  a  principal 
maximum  of  cloud  about  noon  or  a  little  after,  and  a  prin- 
cipal minimum  in  the  night.  This  varies,  however,  not 
only  with  change  of  locality,  but  also  at  different  seasons 
of  the  year. 


13* 


ELEMENTARY   METEOROLOGY. 


The  Annual  March  of  Degree  of  Cloudiness  shows  in  gen- 
eral  a  maximum  amount  of  cloud  in  the  late  fall  or  early 
winter,  and  a  minimum  in  the  spring,  but  sometimes  re- 
tarded until  summer  for  relatively  low  lands ;  but  in  the 
high  lands  (the  Alps,  for  instance)  the  maximum  occurs  in 
the  summer,  and  the  minimum  in  the  winter. 

The  maximum  cloudiness  occurs  mostly  in  early  winter, 
because  then  the  average  temperature  is  decreasing  rapidly, 
which  causes  an  increase  in  the  relative  humidity.  The 
minimum  occurs  mostly  in  the  spring,  because  then  the 
average  temperatures  are  increasing  rapidly,  and  this 
causes  the  relative  humidity  to  decrease.  The  average 
monthly  cloudiness  depends,  however,  so  much  on  the 
direction  of  the  winds  with  regard  to  the  supply  of 
moisture,  the  relative  humidity,  and  other  conditions,  that 
a  great  variety  of  phases  are  to  be  met  with. 

The  amount  of  cloud  in  Bombay  varies  from  18%  in  December  to 
91  %  in  July.  In  Cairo,  Egypt,  it  varies  from  33  %  in  December  to  6  %  in 
June.  In  Spitzbergen  it  varies  from  51  %  in  December  to  87%  in  Sep- 
tember. On  the  northern  coast  of  Norway  it  varies  from  68  %  in  January 
to  60  %  in  June. 

The  following  table  shows  the  average  monthly  amount  of  cloud, 
and  consequently  the  annual  march  of  cloudiness,  at  a  few  places  in 
the  United  States  (in  percentage  of  total  cloudiness)  :  — 


J- 

F. 

M. 

A. 

M. 

J- 

J- 

A. 

S. 

0. 

N. 

D. 

Year 

Key  West,  Fla.   .     .     . 

42 

35 

29 

3i 

43 

48 

49 

49 

52 

46 

38 

40 

42 

St.  Paul,  Minn.   .     .     . 

49 

48 

5i 

Si 

52 

49 

42 

45 

48 

5i 

58 

5i 

50 

Salt  Lake  City,  Utah   . 

54 

52 

53 

52 

46 

3i 

3° 

3° 

26 

40 

47 

56 

43 

Prescott,  Ariz.     .    .    . 

28 

26 

29 

26 

17 

13 

32 

33 

*7 

x6 

17 

25 

23 

The  Variation  of  Average  Cloudiness  with  Latitude  is  shown 
by  the  following  table,  in  which  the  cloudiness  is  given  in 
percentages  of  the  whole  sky  visible  at  one  time  :  — 


MOISTURE:    VAPOR,  CLOUD. 
AVERAGE  CLOUDINESS. 


139 


LATITUDE. 

NORTHERN  HEMISPHERE. 

SOUTHERN  HEMISPHEKK. 

JANUARY  . 

JULY. 

JANUARY. 

JULY. 

70° 

55 

59 

^ 

—  - 

65° 

56 

61 

— 



60° 

62 

62 

70 

70 

55° 

59 

61 

70 

67 

50° 

57 

57 

70 

62 

45° 

52 

50 

60    * 

54 

40° 

50 

44 

53 

56 

35° 

46 

41 

48 

52 

30° 

44 

42 

47 

47 

25° 

37 

45 

48 

46 

20° 

37 

50 

& 

43 

15° 

40 

53 

53 

48 

10° 

45 

59 

58 

5i 

5° 

5° 

59 

56 

57 

Equator  o° 

5<> 

58 

-  — 

— 

The  following  conclusions  are  drawn  from  charts  made 
of  the  cloud  distribution.  The  degree  of  cloudiness  is 
arranged  in  zones  parallel  to  the  equator,  The  zones  of 
minimum  cloudiness  are  reached  at  2O°-25°  north  latitude, 
and  about  25°-3O°  south  latitude,  and  the  amounts  are 
least  in  the  northern  hemisphere.  The  region  of  maximum 
cloudiness  is  reached  at  about  latitude  60°  in  the  northern 
hemisphere,  and  then  there  is  a  decrease  toward  the  north 
pole;  but  in  the  southern  hemisphere  there  is  an  increase 
up  to  60°  south  latitude,  which  is  as  far  as  observations 
extend.  These  zones  present  a  well-defined  tendency  to 
follow  the  sun  in  its  variations  in  latitude  for  the  year, 
lying  at  a  higher  latitude  in  summer  than  in  winter. 

Fogs.  —  Fog  at  the  earth's  surface  shields  the  ground  from,  the  solar 
rays  in  proportion  to  its  density ;  and  at  its  upper  limit  it  must  acquire 


140  ELEMENTARY   METEOROLOGY. 

a  temperature  something  like  that  which  would  occur  at  the  earth's 
unclouded  surface.  The  thickness  of  a  fog  is  shown  by  the  length  of 
time  necessary  for  it  to  be  dissipated  by  the  solar  rays.  Above  the 
ground  fog  the  air  is  clear  and  dry,  and  radiation  takes  place  un- 
hindered. Thus  it  happens  that  bright  clear  weather  is  experienced 
after  the  dissipation  of  a  fog,  which  at  first  seems  to  be  a  forerunner 
of  rain. 

Ocean  Fogs. — When  warm  moist  air  is  carried  by  the 
atmospheric  circulation  into  a  cooler  region,  the  moisture 
is  condensed,  and  fog  forms. 

Such  cases  occur  with  marked  frequency  over  the  North  Atlantic 
Ocean  in  the  region  to  the  south  and  east  of  Newfoundland,  and  con- 
sequently directly  in  the  track  of  the  transatlantic  ocean  steamers.  In 
storms,  when  the  wind  blows  from  the  east  or  southeast,  the  warm 
moist  air  from  the  Gulf  Stream  is  blown  over  the  colder  waters  of  the 
Arctic  current  or  over  the  ice  floes  which  this  current  brings  down  from 
the  north,  and  fog  condensation  takes  place.  The  greatest  frequency 
of  fog  occurs  in  those  latitudes  at  about  the  55th  meridian  .(west  of 
Greenwich).  The  number  of  days  with  fog  for  each  month  in  that 
region  is  as  follows:  — 

J.     F.    M.     A.    M.     J.      J.     A.     S.     O.     N.     D.      Year. 
5     ii     12     15     18     17     23     22     15     13     10      4         165 

The  sea  air  contains  more  moisture  in  the  summer  time  than  in  win- 
ter, which,  together  with  the  great  differences  in  temperature  due  to 
the  large  quantity  of  field  ice  and  icebergs  off  Newfoundland,  causes 
the  relatively  great  frequency  of  fogs  during  the  warm  season. 

Maximum  Amount  of  Water  in  the  Air.  —  The  largest 
possible  amount  of  moisture  which  can  exist  at  any  one  time 
in  the  air  (without  the  supporting  power  of  upward  air  cur- 
rents) between  the  earth's  surface  and  certain  altitudes  is 
best  shown  by  the  number  of  inches'  depth  of  rain  which  it 
would  make  if  all  of  the  moisture  were  condensed,  and  fell 
as  rain.  In  the  most  favorable  case  there  would  not  be 


MOISTURE:    VAPOR,  CLOUD. 


141 


two  inches  of  water  between  the  earth's  surface  and  an 
altitude  of  three  miles  above  it. 


DEPTH  OF  WATER  FOR  THE  FOLLOWING  DEW  POINTS  AT 

HEIGHT  OF  COLUMN  OF  AIR 

THE  EARTH'S  SURFACE. 

80°  F. 

>JO°  F. 

60°  F. 

50°^. 

Feet. 

Inches. 

Inches. 

Inches. 

Inches. 

6,OOO 

i-3 

1.0 

0.7 

0.5 

12,000 

2.1 

i-S 

I.I 

0.8 

l8,000 

2.5 

1.8 

i-3 

0.9 

24,000 

2.7 

2.0 

1.4 

I  Jo 

30,000 

2.8 

2.1 

i<5 

I.I 

The  above  table  shows  the  amount  of  water  which  may  exist  in  the 
air  below  certain  altitudes  for  various  temperatures. 


CHAPTER    VI. 
MOISTURE :  PRECIPITATION. 

Precipitation  occurs  when  moist  air  is  cooled  below 
the  dew-point,  the  moisture  taking  the  form  of  rain,  snow, 
hail,  dew,  or  frost,  depending  on  the  conditions  under  which 
the  condensation  takes  place  and  is  maintained.  The 
greater  part  of  the  precipitation  occurs  in  the  form  of  rain 
in  the  regions  of  the  earth  inhabited  by  man,  and  we  shall 
therefore  treat  it  more  fully  than  the  other  forms  of  pre- 
cipitation, which  will  receive  but  brief  mention. 

Precipitation  is  an  intermittent  phenomenon  of  a  common  occurrence, 
and  it  is  therefore  of  importance  to  observe  its  duration  and  frequency 
as  well  as  its  amount.  No  other  important  meteorological  element  ex- 
hibits the  variability  of  rainfall,  and  it  is  next  to  temperature  in  the 
importance  of  its  effects  on  life  on  the  earth. 

Rain.  —  When  clouds  form  over  a  region  in  which  the 
air  is  nearly  or  quite  saturated  with  moisture,  the  globules 
of  water  which  form  the  cloud  unite,  and  descend  through 
the  stratum  of  moist  air  underneath,  and  fall  as  rain  when 
the  temperature  is  above  the  freezing  point.  The  rain- 
drops vary  in  size  from  those  of  the  fine  mist  which  we 
encounter  in  fog,  to  the  large  isolated  raindrops  which  fall 
from  clouds  at  considerable  altitudes. 

The  production  of  rain  has  been  considered  during  the  past  century 
to  be  mainly  the  result  of  the  mechanical  mixture  of  air  masses  of  differ- 
ent temperatures  and  humidities.  Objections  to  this  theory  were  made 
more  than  half  a  century  ago,  but  they  did  not  receive  proper  con- 

142 


MOISTyRE:    PRECIPITATION.  143 

sideration.  It  was  not  until  about  1870,  however,  that  the  opposite 
extreme  view  was  taken,  that  the  mechanical  mixture  of  the  air  would 
never  alone  cause  the  condensation  of  vapor  and  formation  of  rain,  and 
that  this  last  was  entirely  due  to  adiabatic  changes.  Each  of  these 
theories  had  some  truth  in  it,  but  neither  one  of  them  was  satisfactory 
in  itself.  A  few  years  later,  1874,  the  proper  relative  importance  of  the 
two  theories  was  pointed  out ;  and  it  was  shown,  that  while  the  mechan- 
ical mixture  of  air  might  produce  some  slight  rainfall,  yet  the  amount 
would  be  small  compared  with  that  due  to  adiabatic  causes. 

Causes  of  Condensation  and  Precipitation.  —  As  we  now 

view  the  matter,  there  are  three  causes  of  condensation  of 
water  vapor  in  the  atmosphere ;  and  precipitation  may  be 
regarded  as  but  an  ultra  condition  of  condensation. 

1.  The  Cooling  of  the  Air  by  Contact  with  Cold  Bodies  or 
by  Radiation  is  the  most  powerful  and  easiest  carried  on 
of  the  rain-forming  processes,  and   a  very  slight  cooling 
by  this  means  will  produce  rainfall     Very  excessive  rain- 
falls in  the  mountainous  regions  are  due  to  this  process, 
especially  when  assisted  by  the  upward  movement  of  air  in 
vertical  currents. 

If  we  have  a  mass  of  warm  saturated  air  at  a  temperature  of  68°  F.  and 
27.56  inches  pressure,  it  requires  this  air  to  be  cooled  by  radiation  or 
contact  with  a  cold  body,  by  1.5°  F.,  that  is  to  66.5°  F.,  in  order  to 
produce  5  grains  of  precipitation  per  pound  of  air. 

2.  The  Cooling  of  the  Moist  Air  by  the  Adiabatic  Pro- 
cess. —  Dry  air  cools  by  adiabatic  expansion  at  the  rate  of 
about  i°  F.  for  183  feet,  or  0.55°  F,  per  100  feet  of  ascent. 
When  the  air  contains  moisture  which  has  not  yet  reached 
the  point  of  saturation,  this  rate  of  decrease  is  nearly  the 
same  as  for  dry  air.     Where  the  air  is  saturated,  the  cool- 
ing takes  place  at  a  less  rapid  rate, 

If  the  air  is  saturated,  and  at  27.56  inches  air  pressure  and  a 
temperature  of  68°  F.,  it  is  necessary  to  move  it  upward  to  a  height 


144  ELEMENTARY  METEOROLOGY. 

of  1,017  ^et,  by  which  it  will  be  cooled  from  68D  F,  to  65.1°  F.,  in 
order  to  obtain  from  it  the  5  grains  of  precipitation  from  a  pound  of 
the  air. 

Probably  most  of  the  irregular  rainfalls  which  occur  are 
due  to  this  process. 

3.  The  Mechanical  Mixture  of  the  Air  having  different 
temperatures  and  humidities  is  brought  about  by  motions 
of  the  air  in  various  directions. 

The  air  pressure  varies  with  the  altitude ;  but  since  the  components 
of  this  mixture  have  a  common  altitude,  then  they  must  have  practi- 
cally the  same  air  pressure.  These  air  masses  which  mingle  together 
may  have,  however,  a  variety  of  relations  depending  on  the  humidity 
and  temperature.  Perhaps  the  most  interesting  question  connected 
with  this  is  that  concerning  the  amount  of  humidity  necessary  in  one 
or  both  of  the  component  air  masses  at  given  temperatures  in  order 
to  produce  condensation  or  rainfall.  Various  such  combinations  may 
occur.  The  relative  humidity  of  the  mixture  must  exceed  that  of  at 
least  one  of  the  component  air  masses  before  condensation  can  occur. 
When  one  component  is  warm  saturated  air,  and  is  mixed  with  cooler 
air,  the  latter  may  have  quite  a  high  degree  of  dryness,  and  still 
condensation  may  occur.  In  fact,  it  is  found  that  when  saturated 
warm  air  is  mingled  with  unsaturated  cold  air,  condensation  occurs 
much  more  easily  than  when  the  latter  is  saturated. 

The  determination  of  the  conditions  under  which  two  air  masses  of 
given  temperature  and  humidity  must  be  mixed  in  order  to  produce  the 
greatest  amount  of  precipitation  has  been  investigated,  but  the  method 
of  computation  is  too  complex  to  explain  here.  The  extreme  case  for 
the  conditions  cited  under  the  first  and  second  causes  of  condensation 
will  show  the  amount  of  precipitation  by  this  method.  If  we  suppose  two 
air  masses  to  have  an  air  pressure  of  27.5  inches,  and  both  to  be  saturated 
and  have  temperatures  of  32°  F.  and  68°  F.  respectively,  then  the  maxi- 
mum amount  of  precipitation  which  could  fall  from  the  mixture  of  the 
two  air  masses  at  52°  F.  would  be  5  grains  per  pound  of  the  mixture. 

In  many  cases  where  the  one  component  air  mass  is  very 
dry  and  cold,  it  is  impossible  to  obtain  any  precipitation  by 
its  mixture  with  moist  warm  air.  This  mixing  of  the  dif- 


MOISTURE:    PRECIPITATION. 


145 


ferent  air  masses,  while  not  producing  much  precipitation, 
plays  a  very  important  part  in  the  formation  of  clouds,  as 
we  have  seen  on  p.  136. 

The  Amount  of  Rainfall  is  measured  by  the  depth  of 
fallen  water,  expressed  in  inches  or  millimeters.  The  rain 
water  is  caught  in  a  catch  basin  called  a  rain  gauge ;  and 
when  the  depth  of  the  water  is  measured  directly,  this  must 
take  place  in  a  symmetrical  vessel  having  the  same  water 
surface  area  as  the  receiving  mouth  of  the  gauge*  Rain 
fall  data  inchide  snowfall. 


"Front  Vieiv    Vertical  Section 


Rain  gauges  are  usually  ver- 
tically placed  sheet-metal  hollow 
cylinders  of  from  5  to  8  inches 
in  diameter.  Fig.  41  shows  a 
rain  gauge  with  a  funnel-shaped 
mouth,  A,  and  an  inside  receiver, 
C,  of  less  diameter  than  the  outer 
cylinder. 


Horizontal  Sec.  E.F, 


516171319202122232}  tncftaS 


Scale 
FIG    41.  — RAIN  GAUGE. 


The  Diurnal  Change  of 
the  Rainfall  is  very  marked, 
as  to  both  the  amount  and 
the  frequency  of  occurrence 
of  rainfall.  For  both  of  them  there  is  a  chief  maximum 
in  the  warmest  part  of  the  afternoon  at  two  or  three 
o'clock  or  later;  and  a  principal  minimum  in  the  morning, 
two  or  three  hours  before  noon.  A  secondary  maximum 
occurs  frequently  at  about  two  or  three  o'clock  in  the  morn- 
ing, and  a  secondary  minimum  at  about  midnight. 

In  the  higher  latitudes  the  maximum  amount  occurs  at  a  later 
hour  in  summer  than  in  winter ;  but  this  is  not  so  apt  to  be  the  case 
for  the  frequency  of  rainfalL 

The  amount  of  rain  at  the  hours  of  maximum  is  usually 
from  two  to  three  times  that  at  the  hour  of  minimum ;  but 


146  ELEMENTARY   METEOROLOGY. 

in  some  extreme  cases  in  the  tropics,  in  certain  months,  nearly 
all  the  rain  falls  in  the  afternoon,  when  the  amount  may  be 
nearly  twenty  times  that  at  the  morning  minimum.  The 
amplitude  of  oscillation  is  greater  in  summer  than  in  winter. 

The  Maximum  Amount  of  Rainfall  within  a  Day  is  in 
some  instances  enormous.  On  one  occasion  in  Japan  29.5 
inches  of  rain  fell  in  24  hours,  and  in  India  39.5  inches  fell 
in  one  day.  This  is  as  much  as  would  fall  in  a  favorably 
situated  region  in  a  cold  temperate  climate  in  a  year. 

Monthly  and  Annual  Rainfall.  —  The  annual  march  of 
rainfall  is  so  variable  for  different  places,  that  many  dif- 
ferent regions  would  have  to  be  mentioned  separately  in 
order  to  give  a  true  representation  of  it  in  all  its  variety  of 
phases.  The  direction  of  the  wind,  whether  it  blows  from 
a  region  of  moist  air,  such  as  that  over  the  water,  or  from  a 
region  of  dry  air,  makes  an  enormous  difference  in  the 
amounts  of  rainfall  for  the  various  months.  And  when  to 
this  is  added  the  influence  of  elevated  land  or  mountain 
regions,  it  becomes  impossible  to  state  general  laws  gov- 
erning the  annual  distribution  of  rainfall,  depending  on 
differences  of  latitude  and  longitude  merely.  Such  is  the 
effect  of  mountain  ranges  especially,  that  two  neighbor- 
ing places  only  a  few  miles  apart  may  have  totally  dif- 
ferent rainfall  conditions.  When  the  wind  blows  steadily 
from  the  ocean  (which  mainly  supplies  the  air  with  mois- 
ture) towards  a  not  distant  inland  high  range  of  mountains, 
and  the  mountains  lie  across  the  path  of  the  wind  at  right 
angles  to  it,  we  have  the  greatest  differences  in  rainfall 
within  narrow  limits.  On  the  windward  side  the  rainfall 
is  copious,  and  on  the  leeward  side  there  may  exist  a  desert 
where  the  rain  seldom  falls. 

Variation  of  Rainfall  towards  the  Interior  of  a  Continent. 
—  If  any  law  may  be  expressed  concerning  the  conti- 


MOISTURE :    PRECIPITATION.  147 

nental  influence  on  rainfall,  it  is  this:  that  the  rainfall 
decreases,  both  in  quantity  and  frequency,  with  the  dis- 
tance inland  from  the  sea  or  water  surface  in  general. 
But,  as  before  stated,  a  range  of  mountains  may  inter- 
vene, and  utterly  change  the  natural  tendency  to  follow 
this  law. 

Variation  of  Rainfall  with  Latitude.  —  The  total  amount 
of  precipitation  in  the  year  is,  on  the  average,  greatest  in 
equatorial  regions,  and  decreases  towards  the  poles.  It  is 
greatest  at  a  distance  of  a  few  degrees  from  the  equator, 
and  there  is  a  slight  decrease  towards  the  equator. 

The  variation  of  the  average  annual  amounts  of  precipitation  with 
latitude  is  roughly  shown  by  the  following  table,  in  which  this  average 
is  given  in  inches  for  regions  between  each  ten  degrees  of  latitude :  — 

AVERAGE  ANNUAL  RAINFALL  (in  inches). 


90° — 80°  (north)  .    .  4 

80—70        "  „    „  9 

70   60                   "  .         „  23 

6O   5O  o         .  32 

50—40                     "  .o  36 


40° — 30°  (north)  .  .    28 

30—20        "  .  .    48 

20  — 10        "  o  .    92 

10  —  o        "  .  0    90 

o  — 10  (south)  .  .  128 


10° — 20°  (south)  .  .  62 

20—30        "  .  .  42 

30  —  40        "  .  „  30 

40—50         "  o  c  44 

50—60         "  .  .  40 


The  Variation  of  Rainfall  with  Altitude  is  such  that  from 
sea  level  up  to  an  altitude  of  about  4,000  feet  there  is  an  in- 
crease in  the  amount  of  rainfall ;  while  at  greater  altitudes 
it  again  decreases.  The  increase  in  the  rainfall  from  low 
lands  up  to  the  altitude  of  maximum  amount  of  rain  is  from 
two  to  four  fold  the  amount  in  the  low  regions. 

The  influence  of  mountains  makes  itself  felt  by  the 
increase  of  the  rainfall  to  a  considerable  distance  from 
them  on  the  plains  below ;  but  it  is  probable  that  this 
increase  occurs  only  on  the  windward  side  of  the  moun- 
tains, and  that  the  condensation  is  due  to  the  adiabatic 
cooling  of  the  air,  as  it  rises  at  some  distance  from  the 
mountain  in  order  to  cross  it 


148  ELEMENTARY   METEOROLOGY. 

The  effect  of  increase  of  rainfall  with  altitude  is  very  curious  in  some 
dry  regions  where  at  low  altitudes  only  a  small  amount  of  precipitation 
takes  place  ;  and  at  higher  latitudes  on  the  mountain  sides  there  will  be 
such  an  increase  of  rainfall  as  to  form  belts  of  land  where  enough  rain  is 
received  to  cause  a  considerable  or  even  luxuriant  growth  of  vegetation 
in  an  otherwise  arid  region.  Above  this  watered  region  there  may  be 
such  a  decrease  in  the  rainfall  as  to  make  it  impossible  for  plants  to 
grow. 

The  Geographical  Distribution  of  Annual  Rainfall.  —  It 

has  been  estimated  that  about  6%  of  the  land  surface  has 
an  average  annual  rainfall  of  over  75  inches;  16%  has 
from  50  to  75  inches;  about  25%  has  from  25  to  50  inches; 
over  30%  has  from  10  to  25  inches;  and  over  20%  has  less 
than  10  inches.  The  results  of  rainfall  observations  on 
the  ocean  are  unsatisfactory,  owing  to  the  lack  of  perma- 
nent places  of  observation.  The  distribution  of  rainfall 
(precipitation)  over  the  land  surface  is  well  shown  on  the 
accompanying  chart  (Fig.  42). 

The  darkest  shading  shows  where  the  rainfall  is  over  75  inches.  The 
greatest  rainfall  far  exceeds  this  amount,  however,  for  a  number  of  locali- 
ties. Among  the  Chassia  Hills  in  India  the  average  rainfall  is  over  470 
inches,  and  in  some  other  localities  in  the  tropics  the  amount  reaches 
190  inches.  On  the  tropical  islands,  and  especially  those  containing 
elevated  land,  the  rainfall  is  usually  excessive ;  also  on  the  portions  on 
the  west  coast  of  Europe  where  there  are  high  ranges  of  hills,  on  the 
west  coast  of  North  and  South  America  beyond  latitude  40°,  and  on 
the  eastern  coasts  of  Newfoundland  and  Japan,  the  rainfall  is  tropical 
in  amount. 

Copious  rainfall,  that  is,  between  50  and  75  inches,  is  shown  by  the 
next  lighter  shading.  This  is  the  rainfall  for  the  greater  part  of  Central 
Africa  and  South  America,  regions  of  smaller  extent  in  southeastern 
Asia  and  in  southeastern  and  northwestern  North  America,  northwest- 
ern Europe,  portions  of  central  Europe,  and  the  eastern  coasts  of  Aus- 
tralia and  central  Asia,  and  the  higher  eastern  coast  of  North  America. 

The  region  of  moderate  rainfall,  that  between  25  and  50  inches, 
shown  by  still  lighter  shading,  covers  most  of  the  eastern  half  of  North 


(149) 


150  ELEMENTARY   METEOROLOGY. 

America,  central  Europe,  southeastern  Asia,  and  south-central  Africa, 
South  America,  and  Australia.  Northern  Africa  forms  a  transition  zone 
to  regions  of  Jess  moisture. 

The  region  of  light  rainfall  is  that  having  between  10  and  25  inches 
of  rain.  This  covers  an  enormous  territory  in  the  northern  hemisphere, 
the  most  of  the  western  half  and  much  of  northern  North  America, 
northern  Asia,  and  Russia ;  and  it  forms  broad  transition  zones  to  the 
arid  regions  to  be  next  mentioned. 

The  arid  regions,  those  in  which  the  rainfall  is  below  10  inches,  are 
found  in  north-central  and  southwestern  North  America ;  south-central 
and  west-central  South  America ;  southwestern  and  almost  the  whole 
of  northern  Africa ;  southwestern,  central,  and  northeastern  Asia ;  and 
central  and  western  Australia. 

The  general  seasonal  rainfall  distribution  over  the  globe  is  shown 
on  the  accompanying  chart  (Fig.  43).  Within  each  of  these  broadly 
characterized  regions  there  are  minor  subdivisions  having  special  fea- 
tures, but  presenting  great  complexity  of  detail.  These  have  therefore 
been  omitted  on  the  chart.  The  major  subdivisions  are  as  follows  :  — 

1.  (Light  Pink)  Region  of  Tropical  Rainfall:  Principal  dry  season 
in  winter  and  spring;  maximum  rainfall  in  summer. 

2.  (Dark  Blue)  Region  of  Subtropical  Rainfall :  Maximum  in  win- 
ter; summer  rainless. 

1.2.  (Dark  Purple)  Transition  Region  between  i  and  2  :  Rain  win- 
ter and  summer. 

3.  (Medium  Blue)  Region  of  Maximum  Rainfall  in  winter ;  some- 
what less  rain  in  summer. 

4.  (Dark  Pink)  Region  of  Minimum  Rainfall  in  late  summer,  with 
considerable  rainfall  at  other  seasons. 

5.  (Light  Purple)  Region  of  Moderate  Rain  during  all  the  months. 
(Snow  in  winter.) 

6.  (Light  Blue)  Wet  Region  :  Copious  Rains  during  the  whole  year, 
but  most  rain  in  winter. 

7.  (White)  Dry  Region  :  All  months  of  the  year  deficient  in  rainfall. 

Characteristics  of  Rainfall  in  the  Distinctive  Rain  Regions 
of  the  Earth.  —  The  rainfall  areas  have  been  divided  into 
the  subequatorial  or  tropical,  subtropical,  and  temperate  re- 
gions. The  tropical  region  lies  on  each  side  of  the  equator, 
between  the  tropics.  The  subtropical  region  extends  mostly 


152  ELEMENTARY   METEOROLOGY. 

from  about  the  tropics  poleward,  and  reaches  as  far  as  the 
4Oth  parallel  in  many  cases.  The  temperate  region  lies  to 
the  poleward  of  the  tropical  region. 

The  Tropical  Rainfall  occurs  mostly  in  summer,  and  is 
partly  the  accompaniment  of  local  storms,  and  due  to  the 
vertical  air  currents  which  arise  especially  at  the  time  of 
greatest  midday  heat  in  summer,  and  is  partly  due  to  the 
steady  winds  from  the  east  bearing  the  moisture-laden  sea 
air  on  to  the  mountains  of  continents  and  islands.  Also, 
as  in  India,  the  monsoon  winds  bring  a  rainy  season  when 
they  blow  from  the  ocean  landwards ;  but  it  requires  a 
range  of  mountains  to  produce  summer  rains  in  the  path 
of  these  steady  air  currents. 

There  is  a  shifting  of  the  tropical  or  subequatorial  rain 
belt  with  the  shifting  of  the  sun  from  one  hemisphere*  to 
the  other,  because  with  this  there  is  also  a  shifting  of  the 
equatorial  wind  system.  In  the  region  of  calms  (the  dol- 
drums) occur  mainly  the  rains  due  to  the  local  upward 
currents  ;  and  the  shifting  of  this  region  follows  the  annual 
course  of  the  sun  in  the  heavens.  In  the  region  over 
which  the  doldrums  pass  twice  in  the  year  there  are  two 
rainy  seasons,  with  two  dry  seasons  between. 

The  excessive  rainfalls  of  the  eastern  coasts  and  moun- 
tain slopes  near  the  equator  are  due  to  the  steady  winds 
carrying  the  moisture  from  the  oceans  to  the  land ;  but 
where  the  excessive  rainfalls  occur  on  the  western  coasts, 
they  are  due  to  the  action  of  vertical  air  currents  in  local 
storms,  because,  when  the  winds  have  passed  over  the 
continents  from  the  east,  they  have  been  deprived  of 
much  of  their  moisture.  The  amount  of  water  usually 
existing  in  the  air  as  vapor  is  very  great  in  the  equa- 
torial region,  owing  to  the  high  temperatures  prevailing 
there. 


MOISTURE:    PRECIPITATION. 


153 


Long-continued  series  of  rainfall  observations  have  been  made  at 
but  few  places  in  the  tropical  rain  belt,  outside  of  India,  but  the  follow-' 
ing  table  shows  the  average  monthly  and  annual  rainfall  at  some  selected 
stations.  These  show  the  rainfall  conditions  on  both  the  eastern  and 
western  coasts  of  the  continents. 

TROPICAL  OR  SUBEQUATORIAL  RAINFALL  (in  inches). 


LATI- 

T. 

F. 

M. 

A. 

M. 

T. 

T. 

A. 

R. 

0. 

N. 

D. 

Yn 

AFRICA. 

TUDE 

Loando  (west  coast)   . 

9°S. 

2.4 

i.i 

i-3 

3-3 

o-3 

o.o 

O.O 

0.0 

O.I 

0.2 

2-5 

1.2 

12.4 

Gaboon          " 

0° 

7-5 

9.0 

15.0 

8.7 

8.9 

0.9 

0.5 

1.2 

8.1 

19.4 

19.1 

7-5 

105.8 

St.  Louis       "          .  . 

i6°N. 

0.2 

0.5 

0.0 

0.0 

0.2 

0.4 

2.6 

8.0 

3-7 

o-5 

0.0 

0.0 

16.1 

Port  Louis  (east  coast) 

2°S. 

5-7 

n.8 

5-2 

3-i 

2.1 

i-5 

0.9 

i-5 

0.4 

0.7 

i-7 

3-7 

38.3 

TROPICAL  AMERICA. 

Rio  Janeiro  (east  coast) 

23°  S. 

5-4 

4-7 

5-9 

3-4 

4.8 

1-5 

i-3 

2.8 

3-3 

3-9 

5-7 

5-2 

47-9 

Georgetown         " 

7°N. 

6.9 

5-8 

7-3 

7-3 

I4.I 

i3-9 

II.O 

7-4 

2.6 

2-5 

5* 

10.8 

95.1 

Havana 

23°  N. 

3-3 

1.6 

i.S 

3-2 

4.1 

5-7 

4-9 

4.8|  6.0 

6.7 

2.2 

2.2 

46.3 

Quito  (west  coast)   .  . 

o° 

3-2 

6.4 

4.4 

6.6 

5-i 

2-5 

1.4 

1.8 

1.8 

4.4 

6.2 

2.8 

46.6 

ASIA, 

i°  N 

R   1 

fi  T 

6  8 

7.0 

6  3 

7.2 

6.2 

9.1 

7pl 

8  a 

10  8 

22°  N 

i  6 

q.c 

17  2 

14  2 

ofi 

84.7 

Calcutta  

22°  N. 

0.4 

1.0 

i*3 

2-3 

5-6 

n.8 

13.0 

i3-9 

10  0 

S-4 

0.6 

0.3 

65.6 

Cherrapungi  (Assam) 

0.6 

2.6 

9.0 

29.6 

50.0 

IIO.O 

120.5 

78.957.1 

136 

1.8 

o-3 

474.0 

It  is  seen  that  in  most  cases  the  excessive  rainfall  occurs 
during  but  a  few  months  of  the  year. 

The  Subtropical  Rainfall  is  characterized  by  a  winter 
rainy  season  and  summer  dryness.  The  total  rainfall  is 
relatively  small,  and  is  from  three  to  ten  times  as  much  dur- 
ing the  colder  half  year  as  during  the  warmer  six  months, 
In  some  of  the  regions  (as,  for  instance,  in  Egypt  and  Pal- 
estine) there  is  scarcely  any  rainfall  during  the  summer 
months.  This  region  stretches  up  to  nearly  the  4Oth  de- 
gree of  latitude  over  the  ocean,  but  is  very  variable  on  the 
continents,  owing  principally  to  the  modifying  circumstances 
of  mountain  ranges  and  winds.  In  the  neighborhood  of 
the  Mediterranean  Sea  there  is  a  decrease  in  rainfall  from 
north  to  south,  and  from  the  west  towards  the  east. 


154 


ELEMENTARY   METEOROLOGY. 


The  following  table  shows  characteristics  for  this  region,  where 
the  lands  bordering  on  the  Mediterranean  Sea  give  the  best  examples 
of  the  type :  — 

RAINFALL  IN  SUBTROPICAL  REGION  (in  inches). 


T 

F 

M 

A 

M 

T 

T 

A 

s 

O 

N 

D. 

Y 

L'aguna,  Teneriffe  .  . 
Madeira. 

9.6 
6  \ 

5-7 

2  Q 

6.1 
^6 

2.2 

I  e 

1-3* 

I  2 

0.4 
06 

0.0 
O  O 

o.o 

O  3 

0.4 

I  2 

2.2 

"6 

4.8 

c  c 

10.9 

43-6 

Sahara  

I.O 

I.I 

I  7 

I  °. 

06 

O  2 

O  ^ 

I  2 

08 

I  O 

12  2 

France  (south  coast) 
Italy  (south  coast)  . 
Beirut  . 

2-5 
3-i 

c.4 

2.0 

2-5 

7.2 

J-7 

2.8 
4.0 

1-7 

2.8 
o.q 

2.2 
2.2 
O.7 

I.O 

i-3 
0.4 

o-5 
0.6 
o.o 

I.O 

,6 
o.o 

3-o 

2-5 
0.7 

4.0 

3-8 

T  8 

34 
4.1 

4-7 

1-7 
4.1 
7.6 

24.8 
31-5 

q62 

Alexandria 

2  I 

I  7 

I  O 

O  I 

O  O 

O  O 

O  O 

O  O 

O  I 

O  2 

I  7 

I  Q 

8  7 

The  subtropical  rains  occur  in  that  region  in  which  the 
annual  shifting  of  the  atmospheric  circulation  with  the 
course  of  the  sun  brings  into  play  in  the  summer  the  dry 
trade  winds  blowing  from  the  east ;  and  in  the  winter  the 
general  winds  from  the  west,  which  are  rain-bearing,  be- 
cause in  that  great  western  current  occur  rain-producing 
storms  (the  cyclones  of  lower  middle  latitudes,  to  be  de- 
scribed later).  When  the  trade  winds  from  the  east  are 
present,  the  season  is  dry ;  when  the  winds  from  the  west 
are  present,  the  season  is  wet.  The  stormy  winds  from 
the  west  make  the  west  coast  in  this  region  very  wet 
during  the  rainy  season,  because  they  bring  the  moist  air 
from  the  ocean  to  the  land.  On  the  east  coast  the  winds 
from  the  east  may  still  be  the  most  rainy,  because  they 
bring  the  moist  air  to  the  land,  and  the  west  winds  have 
been  deprived  of  much  of  their  moisture  in  crossing  the 
continent. 

Rainfall  of  the  Temperate  Region.  —  In  the-  temperate 
region,  which  is  to  the  poleward  of  the  subtropical, 


MOISTURE:    PRECIPITATION.  155 

there  is  usually  rain  in  all  the  months  of  the  year,  but 
the  amount  is  not  usually  distributed  evenly  over  all 
seasons. 

The  rainfall  causes  are  very  complex  in  this  region.  It 
is  the  region  of  the  greatest  number  of  storms  which  occur 
in  the  strong  permanent  air  current  from  the  west.  These 
storms  are  most  frequent  in  the  cold  season,  which  in- 
creases the  amount  of  rainfall  at  that  time  of  year.  In 
winter  time,  too,  winds  blowing  the  moisture-laden  air  from 
the  oceans  landward,  carry  it  from  a  warmer  to  a  colder 
region ;  and,  the  temperature  of  the  air  being  lowered, 
moisture  is  condensed,  and  falls  as  rain.  In  the  summer 
time  the  interiors  of  the  continents  become  greatly  heated, 
and  violent  local  storms  arise,  which  cause  an  increase  of 
summer  rains. 

In  western  Europe  the  least  rainfall  occurs  in  midsum- 
mer, and  the  greatest  in  the  fall  or  winter;  in  central 
Europe  the  minimum  occurs  in  the  early  spring  or  win- 
ter, and  the  maximum  in  summer ;  in  Russia  and  Sibe- 
ria the  maximum  occurs  in  summer,  and  the  minimum 
in  winter. 

In  North  America,  on  the  Pacific  coast,  there  is  a  winter 
maximum  of  rainfall,  and  an  almost  rainless  summer ;  for 
the  Mississippi  River  valley  there  is  a  summer  maximum, 
and  a  minimum  in  the  winter  (but  sometimes  in  the  spring 
or  fall) ;  for  the  eastern  coast  of  the  United  States  there  is 
a  late  summer  maximum  and  early  summer  minimum  for 
the  middle  regions,  a  winter  maximum  and  early  summer 
minimum  at  the  north,  and  at  the  south  a  late  winter  maxi- 
mum and  late  summer  minimum. 

In  South  America,  on  the  west  coast,  there  is  a  winter 
maximum  and  summer  minimum ;  in  the  interior  this  is 
reversed ;  and  on  the  east  coast  there  is  a  winter  minimum. 


156  ELEMENTARY   METEOROLOGY. 

On  the  South  African  coast  there  is  a  winter  maximum 
and  a  summer  minimum,  but  in  the  interior  there  is  an 
autumn  maximum  and  winter  minimum. 

In  Australia,  on  the  eastern  coast,  there  is  a  summer  or 
early  autumn  maximum  and  winter  minimum,  and  in  the 
interior  a  very  irregular  rainfall.  In  south  and  west  Aus- 
tralia there  is  a  winter  maximum  and  early  autumn  or  sum- 
mer minimum. 

In  the  northern  Arctic  regions  there  is  a  late  spring  or 
early  summer  minimum  and  a  fall  maximum,  except  north 
of  Asia,  where  there  is  a  winter  minimum  and  summer 
maximum,  as  for  Siberia. 

The  Intensity  of  Rainfall  is  obtained  by  dividing  the 
total  rainfall  by  the  number  of  days  on  which  rain  falls. 
The  intensity  is  calculated  for  each  month  and  for  the 
year.  The  distribution  of  the  intensity  of  rainfall  for  the 
months  follows  more  closely  the  amount  of  rainfall  than 
it  does  the  number  of  rainy  days.  In  normal  cases  the 
intensity  is  greatest  in  the  warm  seasons,  but  this  does  not 
always  so  occur. 

Duration  of  Rainfall.  —  The  average  number  of  hours 
per  .day  during  which  rain  falls  is  quite  variable  for  dif- 
ferent regions,  not  only  for  the  year,  but  for  the  different 
months. 

The  greatest  average  duration  of  the  rainfall  occurs  in  the  cold 
season  of  the  year  in  normal  cases  in  the  middle  latitudes.  On  the 
coast  of  Norway  the  average  number  of  hours  is  1 1  :  in  southeastern 
England,  5  ;  in  the  northeastern  quarter  of  the  United  States,  5  ;  in 
the  southeastern  quarter,  4;  in  the  dry  region  in  southwestern  United 
States,  2.5  ;  and  in  the  dry  Rocky  Mountain  region,  about  3. 

The  Variability  of  the  Rainfall  is  obtained  by  dividing  the 
average  deviation  of  the  individual  cases  from  the  average 
rainfall,  by  the  average  amount  of  rainfall.  The  variability 


MOISTURE:    PRECIPITATION.  157 

of  rainfall  (expressed  in  percentage  of  the  average  amount) 
in  general  increases  with  decrease  of  the  absolute  amount 
of  rainfall.  In  regions  of  moderate  rainfall  (30  inches  per 
year)  50  years  of  observations  may  leave  a  possible  error 
of  from  5%  to  10%  in  the  average  annual  rainfall. 

The  variability  in  the  number  of  days  of  rainfall  in  the 
year,  and  for  different  months,  is  found  to  be  the  least  for 
regions  having  the  fewest  number  of  days  with  rainfall, 
and  greatest  for  those  having  the  greatest  number  of  days 
with  rainfall. 

The  occurrence,  during  the  same  months,  of  the  max- 
imum or  the  minimum  phases  of  rain  amount,  rain 
frequency,  and  rain  intensity,  is  rarely  found  in  the 
continental  regions  of  the  temperate  zone. 

The  Probability  of  Rainfall  is  obtained  by  dividing  the 
number  of  rainy  days  by  the  number  of  all  the  days  during 
a  chosen  period,  as  a  month  or  a  year.  This  is  quite  an 
important  element  to  be  considered  concerning  the  rain- 
fall. The  variation  is  so  great  from  region  to  region,  or 
even  within  narrow  limits,  that  of  course  the  phases  of 
maximum  and  minimum  frequency  cannot  be  pointed  out 
unless  special  mention  is  made  of  a  great  number  of 
regions.  It  does  not  necessarily  always  happen  that  the 
greatest  frequency  of  rainfall  occurs  with  the  greatest 
amount  of  rainfall. 

Long-Period  Fluctuations  of  Rainfall.  —  There  is  prob- 
ably a  periodic  oscillation  in  the  amount  of  rainfall  extend- 
ing over  a  period  of  35  years.  During  about  17  years  the 
rainfall  increases  slightly  in  amount,  and  then  decreases 
again  through  the  same  length  of  time. 

We  have  record  of  rainfall  observations  going  back  over  200  years 
at  some  places.  The  records  for  this  century  are  more  accurate  and 
more  numerous  than  those  for  the  last  century,  and  show  for  the  land 


15$  ELEMENTARY   METEOROLOGY. 

the  following  periods  of  excess  and  deficiency  of  rainfall  above  and 
below  the  average  amount :  1815,  excess  of  rainfall ;  1831-35,  deficiency  ; 
1846-50,  excess;  1861-65,  deficiency  ;  1876-80,  excess.  What  the  cor- 
responding conditions  were  over  the  oceans,  we  do  not  know. 

The  Amount  of  Oscillation  of  the  Rainfall  during  the 
probable  thirty-five  year  period  is* of  as  great  interest  as 
the  times  of  oscillation.  The  average  oscillation  is  best 
expressed  in  terms  of  the  average  amount  of  rainfall.  In 
Europe  the  oscillation  is  about  15%;  in  Asia,  30%;  in 
North  America,  25  %  ;  in  Central  and  South  America, 
25  %  ;  the  average  for  all  the  regions  being  about  25  %, 
or  one  fourth  of  the  average  total  rainfall. 

It  has  been  found  that  intensity  of  the  oscillations  increases  towards 
the  interior  of  the  continents.  Thus  in  central  Siberia  2.3  times  as 
much  rain  falls  in  the  period  of  excess  of  rain  as  in  the  period  of  de- 
ficiency, while  in  England  it  is  but  1.2  times  as  much.  Observations 
along  the  coast  of  the  Atlantic  Ocean  have  shown  that  there  is  a  ten- 
dency towards  a  minimum  of  rainfall  at  the  period  of  maximum  rainfall 
in  the  interior  of  the  continent ;  and  this  suggests  that  on  this  ocean, 
at  least,,  there  may  exist  a  compensatory  rainfall  oscillation  the  reverse 
of  that  for  the  land. 

There  is,  during  these  long-period  oscillations  of  rainfall,  a  shifting 
back  and  forth  of  the  rainfall  areas,  or,  as  the  matter  may  best  be 
expressed,  a  shifting  of  the  lines  of  equal  rainfall.  A  rough  computa- 
tion of  this  shifting  of  the  regions  of  equal  rainfall  oceanwards  and  land- 
wards shows  that  the  region  of  24  inches  rainfall  lies  in  Euro-Asia  1,000 
miles,  and  in  North  America  700  miles,  farther  inland  in  the  period 
of  excess  of  rainfall  than  it  does  in  the  period  of  deficiency.  The 
oscillations  in  the  dry  regions  of  Siberia  are  enormous.  The  rain- 
fall which  occurs  in  \\^est  Siberia  in  the  relatively  wet  period  is  only 
to  be  met  with  at  a  distance  of  2,000  or  3,000  miles  from  that  region 
during  the  relatively  dry  period.  The  oscillations  are  least  on  the 
coast,  and  increase  towards  the  interior  of  continents. 

The  Permanent  Increase  or  Decrease  of  Rainfall  in  special 
localities,  due  to  the  increase  or  decrease  in  the  forests,  has 


MOISTURE     PRECIPITATION  159 

oeen  the  subject  of  much  investigation,  and  it  has  not  as 
yet  been  satisfactorily  settled,  Experiments  for  the  arti- 
ficial production  of  rain  by  means  of  explosions  high  in  the 
air  are  not  likely  to  be  successful,  as  no  appreciable  amount 
of  rain  could  be  produced  in  this  manner,  unless  ascending 
air  currents  could  be  caused  and  maintained  for  some  time. 

HaiL — When  raindrops  become  frozen  in  their  passage 
through  the  air,  they  fall  as  hail.  Sometimes  the  rain- 
drops may  be  frozen  in  their  downward  passage ;  but  it  is 
believed  that  they  are  usually  frozen  by  being  first  carried 
upward  by  vertical  air  currents  into  regions  where  the  tem- 
perature is  below  freezing,  and  that  they  then  descend  to 
the  earth  before  they  have  time  to  melt.  Sometimes  hail- 
stones have  a  soft  snowy  center ;  and  frequently  they  have 
several  successive  coats  or  shells  of  ice,  probably  due  to 
their  being  subjected  a  number  of  times  to  alternate  tem- 
peratures above  and  below  freezing. 

SnoWc  - —  When  condensation  takes  place  at  a  tempera- 
ture below  freezing,  minute  ice  crystals  instead  of  water 
globules  are  formed,  and  the  union  of  these  crystals  gives 
us  snowflakes.  Snow  crystals  assume  a  great  variety  of 
beautiful  hexagonal  forms,  some  of  which  are  shown  in 
the  accompanying  figure  (Fig.  44).  While  the  condition 
of  freezing  is  necessary  for  the  formation  of  snowflakes 
in  the  air  above,  yet  they  frequently,  by  falling  rapidly 
without  melting,  reach  the  ground  when  the  temperature 
of  the  lower  air  is  much  above  the  freezing  point  of  water. 

Latitudinal  Limit  of  Snowfall  near  Sea  Level .  —  The 
temperature  of  the  lower  air  increases  from  a  temperature 
at  the  poles  much  lower  than  that  required  for  snowfall,  to 
a  temperature  at  the  equator  much  higher  than  that  at 
which  snow  can  exist.  So  there  must  be  some  region  be- 
tween the  two  where  snow  ceases  to  fall ;  and  to  the  pole- 


(i6o) 


FIG.  44.  —  SNOW  CRYSTALS. 


MOISTURE:    PRECIPITATION.  l6l 

ward  of  this  region  the  length  of  the  season  during  which 
snow  falls  increases  towards  the  poles,  while  on  the  equa- 
torial side  snow  never  falls. 

The  equatorial  limit  of  snowfall  is  in  general  on  the 
continents  at  about  the  Tropics  of  Cancer  and  Capricorn, 
or  23  J°  of  latitude  ;  but  in  western  South  America  it  reaches 
much  nearer  to  the  equator.  Over  the  oceans  the  latitude 
of  35°  is  about  the  limit. 

It  is  found  that  in  very  cold  weather  snow  does  not  fall ;  and  as  there 
must  be  a  limit  to  the  height  of  Lhe  temperature  at  which  snowrlakes 
can  exist,  so  there  must  be  some  intermediate  temperature  of  maximum 
snowfall.  The  subject  has  not  been  very  extensively  investigated,  but 
some  observations  in  the  mountains  of  Germany  have  shown  that,  there 
at  least,  the  snows  occurred  between  the  limits  of  48°  and  8°  F.  for 
the  lower  air  temperature,  with  an  average  temperature  of  30°  F. 

Measurements  of  Snowfall  are  first  made  by  means  of 
a  graduated  rod,  with  which  the  depth  of  the  snow  is 
measured  at  a  number  of  places  which  are  protected  from 
the  wind,  and  yet  freely  accessible  to  the  snow.  The  snow 
in  and  over  the  rain  gauge  must  then  be  melted,  and  the 
amount  of  the  resulting  water  determined  by  the  usual 
method  of  measuring  rainfall. 

The  average  density  of  recently  fallen  snow  may  be  taken  as  about 
o.io  (or  io  inches  of  snow  make  i  inch  of  water)  ;  but  snow  is  lighter 
(less  dense)  when  it  first  falls  than  after  it  has  stood  for  a  while,  lighter 
in  cold  than  in  warm  weather,  lighter  in  woods  than  on  an  open  plain, 
lighter  in  gentle  than  in  strong  wind,  and  the  upper  layers  of  fallen 
snow  are  lighter  than  the  lower.  The  density  of  snow  may  vary 
from  0.02  to  0.9  (the  density  of  ice),  the  density  of  water  being  taken 
as  i  .00 ;  the  conditions  causing  this  great  variation  being  the  hu- 
midity of  the  air,  the  temperature,  the  wind  velocity,  the  immediate 
locality  whence  the  snow  is  taken  (whether  from  drifts  or  protected 
places),  the  time  that  the  snow  has  lain  on  the  ground,  the  form  of 
the  snowflakes,  the  layer  of  snow  chosen,  etc.  In  general,  in  a  cold 


1 62  ELEMENTARY    METEOROLOGY. 

climate  it  may  be  assumed  that  the  fallen  snow  is  lightest  in  the  early 
winter  (in  Siberia,  0.15),  moderately  dense  in  midwinter  (in  Siberia, 
0.2),  and  most  dense  in  spring  (in  Siberia,  0.3). 

Dew.  —  Just  as  soon  as  the  temperature  of  the  surface  of 
the  ground  (or  of  any  other  surface)  falls  below  the  dew- 
point  of  the  adjacent  air,  the  latter  gives  up  part  of  its 
vapor  in  the  form  of  small  water  drops,  or  dewdrops  as 
they  are  called,  which  are  deposited  on  the  cooled  surface. 
When  the  temperature  of  the  surface  lies  below  the  freez- 
ing point  of  water  (o°  C.  or  32°  F.),  then  the  dew  is  de- 
posited in  small  ice  crystals,  and  it  is  called  hoarfrost. 
On  account  of  the  rapid  cooling  by  radiation,  especially 
on  clear  nights,  the  temperature  of  the  ground  and  other 
solid  substances  becomes  colder  than  that  of  the  air  above, 
and  the  dew-point  and  even  frost  point  are  reached  by  the 
ground  and  the  adjacent  air  layer,  while  the  air  at  the 
height  of  a  few  feet  above  the  ground  has  a  temperature 
several  degrees  above  those  points. 

It  has  been  supposed  that  in  very  dry  rainless  regions  plants  depend 
in  a  great  measure  on  the  dew  deposit  for  their  supply  of  water,  but 
the  amount  of  moisture  obtained  in  this  way  is  quite  small. 

In  central  Europe  it  was  found  that  scarcely  one  inch  depth  of  dew 
was  deposited  during  a  year,  which  is  about  3  %  of  the  rainfall. 

Night  Frosts  occur  when  plants  are  exposed  to  tempera- 
tures under  the  freezing  point,  and  the  conditions  are 
much  the  same  as  for  hoarfrost  formation.  Since  by  the 
condensation  of  vapor  as  dew,  latent  heat  is  given  out  and 
thus  retards  the  cooling,  the  temperature  during  the  dew- 
fall  does  not  go  much  below  the  dew-point;  so  that  when 
the  dew-point  lies  above  the  freezing  point,  frost  is  hardly 
to  be  expected;  but  when  the  dew-point  lies  below  the 
freezing  point,  frost  may  be  expected  on  clear  nights.  The 
drier  the  air,  the  more  likely  will  be  the  dew-point  to  be 


MOISTURE:    PRECIPITATION.  163 

below  the  freezing  point,  and  therefore  the  more  likelihood 
of  a  frost.  This  is  seen  on  a  large  scale  in  the  fact  that 
frosts  occur  more  frequently  at  inland  than  at  coast  sta- 
tions. 

Evaporation  of  Water.  —  A  portion  of  the  water  which 
is  found  on  the  earth's  surface  is  given  off  to  the  overlying 
air  by  the  process  of  evaporation.  The  rapidity  with 
which  this  evaporation  takes  place  varies  not  only  with  the 
surface  itself,  whether  it  be  a  free  water  surface  or  wet 
ground  or  vegetable  growth,  but  also  with  the  tempera- 
ture, the  relative  amount  of  water  already  in  the  air,  the 
motion  of  the  air,  and  the  atmospheric  pressure.  In  any 
case  the  amount  of  water  evaporated  varies  directly  with 
the  amount  of  surface  from  which  the  evaporation  takes 
place. 

There  is  great  variability  as  to  the  rapidity  of  evapora- 
tion from  the  ground ;  but  it  may  be  said  that  the  smaller 
the  size  of  the  particles  of  earth,  the  more  rapid  will  be  the 
evaporation,  since  the  capacity  of  the  ground  for  holding 
water,  and  the  capillary  conduction  of  the  water  to  the 
surface,  increase  with  the  fineness  of  the  earth  particles. 
From  a  hard-packed  clay  ground  surface,  almost  no  evapo- 
ration takes  place. 

The  amount  of  evaporation  from  plants  is  enormously 
great.  The  passage  of  water  through  a  plant  from  the 
ground  to  the  free  air  is  called  transpiration.  Five  times 
as  much  water  has  been  transpired  and  evaporated  from 
a  plant  as  from  a  water  surface,  and  more  than  twelve 
times  as  much  as  from  an  ordinary  land  surface  during 
the  same  time. 

A  short  series  of  experiments  has  shown,  that,  accord- 
ing to  the  observed  temperatures  of  the  dew-point  and  the 
surface  of  the  snow,  the  evaporation  from  the  snow  ex- 


164 


ELEMENTARY   METEOROLOGY. 


ceeds  in  amount  the  condensation  from  the  air  by  means 
of  its  cold  surface. 

As  a  standard  for  measuring  evaporation,  the  amount 
evaporated  from  a  freely  exposed  but  shaded  surface  of 
pure  water  is  chosen;  and  the  amount  of  water  lost  by 
evaporation  is  calculated  by  first  measuring  or  weighing 
the  whole  amount  when  first  exposed,  and  then  that  which 
remains  in  the  vessel.  Pure  water  evaporates  probably 
25  %  more  rapidly  than  salt  water. 

A  very  convenient  instrument  for  measuring 
the  amount  of  evaporation  during  the  warm  sea- 
son is  the  Piche  evaporimeter,  and  it  can  be 
mounted  beside  a  thermometer  and  read  with 
as  much  ease  as  one.  This  instrument  (shown 
in  Fig.  45)  consists  of  a  graduated  glass  tube 
closed  at  one  end.  The  tube  is  filled  with 
water;  and  over  the  open  end  is  placed  a  cir- 
cular disk  of  porous  paper,  with  a  very  small 
opening  at  the  center.  The  instrument  is 
mounted  with  the  porous  paper  downward,  and 
this  last  always  presents  to  the  air  a  wet  surface 
from  which  the  water  is  evaporated.  As  evapora- 
tion goes  on,  the  upper  part  of  the  tube  becomes 
emptied  of  the  water,  and  the  air  makes  its  way 
up  through  the  water  column  to  fill  the  vacated 
space.  The  variation  of  the  height  of  the  water 
in  the  tube,  as  read  off  from  the  graduated  scale, 
shows  the  amount  of  water  evaporated  from  the 
tube.  Since,  however,  disks  of  various  degrees 
of  porosity  offer  different  resistances  to  the  pas- 
sage of  the  water,  the  instrument  must  be  compared  with  a  standard 
evaporimeter,  or  a  disk  of  known  porosity  must  be  used. 


FIG.  45.  —  PICHE  EVAPO- 
RIMETER. 


Water  when  evaporated  becomes  a  vapor  which  tends 
to  distribute  itself  through  the  atmosphere  somewhat  after 
the  manner  of  the  air  in  an  empty  space.  Vapor-laden 


MOISTURE:    PRECIPITATION.  1 65 

air  is    also   transferred    by  the    atmospheric    currents   to 
regions  where  there  is  a  less  amount  of  vapor. 

Periodic  Variations  in  Evaporation.  —  The  amount  of 
evaporation  has  a  daily  and  annual  march  following  that 
of  the  temperature.  The  evaporation  is  least  a-t  night, 
and  greatest  shortly  after  noon.  The  amplitude  of  daily 
variation  is  greatest  in  the  interior  of  continents,  and  least 
on  the  coasts.  The  evaporation  during  the  day  is  several 
times  greater  than  during  the  night.  During  the  year  the 
least  evaporation  is  in  midwinter,  and  the  greatest  in  mid- 
summer. 

At  Nukuss  (continental  exposure  in  central  Euro-Asia),  the  evapo- 
ration in  January  was  about  I  inch,  and  in  July  nearly  12  inches.  At 
St.  Petersburg  (seacoast),  however,  the  evaporation  was  less  than  0.2 
of  an  inch  in  January,  and  less  than  2.5  inches  in  July. 

The  amplitude  of  annual  variation  increases  with  the 
latitude  and  towards  the  interior  of  continents. 

The  Average  Annual  Amount  of  Evaporation  varies 
greatly,  but  in  general  it  decreases  with  the  latitude.  In 
the  tropics,  from  a  water  surface,  it  amounts  to  perhaps 
90  inches ;  in  latitude  40°,  to  perhaps  30  inches ;  and  in 
polar  latitudes,  to  10  inches.  In  hot  desert  regions,  as  in 
southwestern  United  States,  it  reaches  even  1 50  inches. 

The  amount  of  evaporation  possible  over  the  land  is 
greater  than  the  amount  of  rainfall  in  most  regions  of  the 
earth ;  but  this  amount  of  water  does  not  evaporate  from 
the  ground,  because  there  is  not  always  sufficient  moisture 
at  the  surface  to  keep  up  the  rate  of  evaporation  which 
could  take  place. 

WALDO   METEOR .  —  IO 


CHAPTER   VII. 
ATMOSPHERIC  OPTICS  AND  ELECTRICITY. 

Luminous  Atmospheric  Phenomena.  —  There  are  a  num- 
ber of  luminous  atmospheric  phenomena  which  come  to 
our  notice,  and  which  it  will  be  convenient  to  mention  at 
this  stage.  They  are  principally  of  two  classes.  The  one 
pertains  to  the  optical  effects  produced  by  and  in  the  at- 
mosphere ;  the  other  concerns  the  electrical  condition  of 
the  atmosphere. 

Transparency  of  the  Air.  —  Any  substance  that  one  can 
see  through  is  said  to  be  transparent.  But  some  sub- 
stances are  more  transparent  than  others;  that  is,  we  can 
see  through  greater  thicknesses  of  some  substances  than 
of  others,  and  with  greater  distinctness. 

We  call  air  a  transparent  substance,  but  the  degree  of  transparency 
is  not  constant.  It  varies  with  the  density  of  the  air  and  the  amount 
and  form  of  moisture  which  it  contains.  Thus  at  sea  level,  and  for 
warm  moist  air,  the  transparency  is  least,  while  on  high  mountains 
with  cool  dry  air  the  transparency  is  greatest. 

Atmospheric  Optics.  —  Light,  in  passing  through  the  air, 
is  subject  to  reflection,  refraction,  diffraction,  and  absorp- 
tion ;  and,  as  a  result  of  these,  there  arise  a  number  of 
phenomena  which  must  be  briefly  mentioned. 

Refraction  of  Light  is  the  bending  of  light  rays.  Light, 
in  passing  from  a  rarer  to  a  denser  medium,  is  bent  from 
its  straight  course  so  that  the  angle  which  the  ray  of  light 

1 66 


ATMOSPHERIC  OPTICS   AND   ELECTRICITY.  l6/ 

makes  with  the  perpendicular  to  the  boundary  surface  is 

less  in  the  denser  medium  than  in  the  rarer  medium ;  and 

where  it  passes  from  a  denser  to  a  rarer  medium,  the  angle 

which  the  ray  makes  with  the 

perpendicular  is  increased.   This 

angle    in    the    first    medium    is 

called    the   angle    of  incidence, 

and  in  the  second  the  angle  of 

refraction. 


In  Fig.  46,  BA  is  the  ray  of  light 
passing  through  a  rare  medium  (as,  for 
instance,  air)  ;  and  upon  its  entrance 
into  a  denser  medium  (as,  for  instance, 
water)  the  ray  will  be  deflected  from 
the  direction  of  its  path  BA,  and  will 
take  the  course  AE.  If  the  line  CD  is  perpendicular  to  the  dividing 
surface  between  the  two  media,  then  BAC  is  the  angle  of  incidence, 
and  DAE  is  the  angle  of  refraction. 

The  amount  of  refraction,  or  the  difference  between  the 
angles  of  incidence  and  refraction,  varies  with  changes  in 
the  density  of  the  transparent  media ;  and  in  the  case  of  light 
entering  the  atmosphere  from  without,  the  rays  are  gradu- 
ally bent  more  and  more  as  the  denser  air  is  encountered. 

The  accompanying  figure  (Fig.  47)  illustrates  the  bending  of  the 
solar  rays  entering  the  atmosphere.  When  the  sun  is  below  the  hori- 
zon, at  C,  it  would  be  invisible  at  A,  on  account  of  the  curvature  of  the 
earth,  if  there  were  no  atmosphere  ;  but  the  solar  rays  entering  the  atmos- 
phere near  the  point  B  are  refracted  so  that  they  reach  A,  and  the  sun 
appears  to  be  at  D,  though  really  at  C  below  the  horizon,  either  in  the 
morning  or  in  the  evening.  So  that,  in  the  polar  regions,  the  sun  is 
visible  while  it  is  in  reality  below  the  horizon,  and  is  thus  seen  earlier 
and  later  during  the  time  of  polar  sunlight. 

The  effect  of  atmospheric  refraction  is  to  slightly  in- 
crease the  amount  of  solar  rays  reaching  a  place. 


1 68 


ELEMENTARY   METEOROLOGY. 


Where  the  light  enters  the  atmosphere  at  a  small  angular  distance 
from  the  zenith,  the  amount  of  refraction  can  be  quite  accurately  com- 
puted, but  near  the  horizon  it  is  very  difficult  to  determine.  For  a 
zenith  distance  of  87.5°  (i.e.,  2.5°  above  the  horizon),  the  refraction 
is  computed  to  be  18.5' ;  but  at  the  horizon  it  is  about  30'. 

The  rays  of  light  coming  from  the  sun  possess  different 
wave  lengths  of  vibration ;  and,  as  these  rays  pass  from 


FIG.  47.  —  REFRACTION  OF  SOLAR  RAYS  BY  THE  ATMOSPHERE. 


one  medium  to  another  of  different  density,  there  is  more 
or  less  of  a  dispersion  of  the  rays,  and  those  possessing 
the  longest  wave  lengths  are  refracted  the  least,  and  those 
with  the  shortest  wave  lengths  are  refracted  the  most ;  so 
that  the  rays  which  enter  the  second  medium  as  white  light 
are  spread  out  by  the  refraction,  and  separated  into  the 
prismatic  band  of  colors,  —  red,  orange,  yellow,  green,  blue, 
indigo,  and  violet,  —  ranged  according  to  the  wave  lengths 
and  refrangibility  of  the  rays.  Where  any  one  color  is 
observed,  it  shows  that  the  rays  with  vibrations  corre- 
sponding to  this  color  are  in  excess  of  any  others  which 
may  be  present. 

Reflection  of  Light  is  the  throwing-back  of  light  rays  from 
the  surface  on  which  they  fall.     When  the  rays  of  light 


ATMOSPHERIC   OPTICS  AND   ELECTRICITY.  169 

strike  a  surface,  they  are  reflected  from  this  surface  again 
in  such  a  direction  that  they  make  the  same  angle  with 
the  perpendicular  to  the  surface  before  and  after  reflection. 

Thus,  if  a  ray  of  light  passes  from  A  towards  B  (Fig.  48),  it  will  be 
reflected  from  B  towards  C\  and  the  angle  ABF,  called  the  angle  of 
incidence,  equals  the  angle  CBF,  called  the  angle  of  reflection.  The 
reflection  of  light  in  the  atmosphere  is  very  important  in  its  effects. 

Reflection  of  Light  from  Dust  Particles.  —  The  reflection 
of  light  from  the  minute  particles  of  dust  in  the  atmosphere 
diffuses  and  scatters  the  light  received  from  the  sun  in  all 
directions,  and  illumines  the 
sky.  This  illumination  of  the 
sky  is  greatest  at  low  altitudes, 
where  the  air  contains  more 
and  larger  dust  particles  than 
at  higher  altitudes. 

Mirage.  —  When  one  layer  of  air  lies  in  contact  with 
another  layer  of  different  temperature  and  density,  the 
boundary  surface  between  the  two,  if  it  is  sharply  defined, 
will  reflect  light  quite  perfectly.  When  the  observer  is 
above  such  a  surface,  he  sees  objects  beyond  and  above 
(but  not  below)  this  surface  reflected  from  it  as  from  a 
horizontal  mirror.  In  case  the  observer  is  below  this 
boundary  surface,  he  sees  the  objects  beyond  and  below 
(but  not  above)  reflected  from  it.  The  images  are  in- 
verted. This  phenomenon  is  called  mirage. 

Diffraction  of  Light  is  the  dispersion  or  breaking-up  of 
light  rays.  When  a  beam  of  light  passes  through  an 
aggregation  of  exceedingly  minute  particles,  or  when  it 
strikes  a  surface  roughened  or  divided  into  very  small 
surfaces,  the  rays  are  broken  up  and  scattered  or  dif- 
fracted. The  interference  of  some  of  these  rays  causes 


I/O  ELEMENTARY   METEOROLOGY. 

some  of  the  color  rays  to  partially,  and  others  to  wholly, 
disappear. 

Colors  of  the  Sky. —  The  light  rays  having  the  more  minute 
wave  length  (those  nearer  the  violet  end  of  the  spectrum) 
are  diffracted  by  the  more  minute  as  well  as  by  the  coarser 
particles,  while  the  rays  of  greater  wave  length  (those 
nearer  the  red  end  of  the  spectrum)  are  diffracted  more 
by  the  coarser  particles ;  and  it  is  on  this  "  selective  "  diffrac- 
tion and  reflection  that  much  of  the  coloring  of  the  sky 
and  other  more  definite  optical  phenomena  depend.  The 
blue  color  of  the  sky  (away  from  the  sun)  is  due  to  the 
more  powerful  diffraction  of  the  blue  rays,  so  that  rays  of 
that  color  are  reflected  towards  the  observer,  while  those  of 
coarser  wave  length  pass  on  and  do  not  reach  him.  As  the 
observer  turns  his  eyes  gradually  towards  the  sun,  the  sky 
loses  its  blue  color,  and  takes  on  a  more  neutral  or  white 
color,  because  the  direct  rays  which  now  reach  him  do  not 
contain  to  any  great  degree  the  blue  component  which  had 
been  previously  reflected  toward  him  from  other  directions. 

When  the  sunlight  passes  through  the  atmosphere,  we 
lose  more  and  more  of  the  colors  having  smaller  wave 
length,  the  thicker  and  denser  the  air  mass  through  which 
the  rays  come,  so  that  the  sun's  rays  reaching  us  contain 
the  most  blue  when  in  the  zenith;  but  as  the  sun  ap- 
proaches the  horizon,  the  rays  at  the  blue  end  of  the 
spectrum  become  scattered,  and  lost  in  the  air,  and  we 
receive  only  those  at  the  red  end,  and  consequently  the 
sun  appears  red  at  the  horizon.  If  the  sun  could  be 
viewed  without  the  intervention  of  an  atmosphere,  it 
would  present  the  distinctly  blue  color. 

The  glow  of  the  sky  which  accompanies  the  rising  and 
setting  of  the  sun  is  due  to  the  diffraction  and  reflection 
of  light  by  the  minute  particles  in  the  air.  As  the  sun 


ATMOSPHERIC   OPTICS   AND   ELECTRICITY  171 

sets  below  the  horizon,  the  intervening  portion  of  the  earth 
cuts  off  the  rays  from  the  lower  atmosphere  (at  the  point 
of  the  observer),  and  the  diffracted  rays  forming  the  glow 
appear  in  the  upper  atmosphere  only.  When  the  particles  of 
matter  in  the  air  are  unusually  numerous  at  high  altitudes, 
the  sunset  glow  of  the  sky  is  extraordinarily  brilliant.  Such  a 
condition  prevailed  during  1883  and  a  few  subsequent  years. 

Corona  and  Halo. — The  sun  or  moon  is  occasionally 
surrounded  by  one  or  more  well-marked  rings  or  circles 
of  light,  but  not  always  of  the  same  diameter  or  color. 
These  rings  are  divided  into  two  classes,  —  the  corona,  of 
small  diameter ;  and  the  halo,  of  greater  extent.  In  the 
corona,  the  color  of  the  inner  part  of  the  ring  is  blue, 
and  of  the  outer,  red.  For  the  larger  halo,  the  order  is 
reversed;  and  red  is  on  the  inside  and  blue  on  the  out- 
side of  the  ring.  The  radius  of  the  halo  is  about  22°, 
45°,  or  90°.  The  corona  may  vary  from  i°  to  10°  or  more 
in  radius. 

The  corona  is  a  diffraction  phenomenon,  while  the  halo 
is  due  to  refraction  and  reflection. 

The  corona  may  consist  of  several  rings  concentric  with 
the  sun  or  moon,  and  occurs  when  mist  or  thin  clouds  par- 
tially obscure  those  luminaries.  Coronae  are  formed  by 
the  diffraction  and  interference  of  light  caused  by  the 
small  water  particles  in  the  cloud.  The  larger  the  water 
particles,  the  smaller  will  be  the  ring ;  and  it  is  when  the 
particles  are  of  different  sizes  that  coronal  rings  of  different 
diameters  exist  at  the  same  time. 

The  halo  occurs  in  connection  with  the  higher  cirrus 
clouds  only.  Halos  are  more  frequently  observed  than 
coronae.  The  faint  halos  are  much  more  easily  detected 
by  observing  in  a  black  mirror  the  reflection  of  the  light 
center  and  its  neighborhood.  There  is  great  variety  in 


172 


ELEMENTARY   METEOROLOGY. 


the  distribution  of  halos,  —  sometimes  the  rings  are  wholly 
separate,  and  sometimes  they  intersect.  The  points  of  in- 
tersection are  unusually  more  or  less  bright  patches,  and 
are  called  mock  suns,  or  sun  dogs  (Fig.  49). 


Mock 

Snn 


FIG.  49.  —  HALO  PHENOMENA. 

In  general,   the   halos   may   be   divided   into   the   three   following 
classes :  — 

1.  Rings  which  have  the  sun  at  the  center,  and  have  a  radius  of 
about  22°.     The  rings  are  about  i°  wide,  and  are  colored  with  the  red 
on  the  inner  side,  where  they  are  sharply  defined,  but  on  the  outer 
violet  side  they  gradually  vanish.     The  space  directly  around  the  sun  is 
bright,  but  towards  the  ring  this  brightness  gradually  disappears. 

2.  Rings  which  pass  through  the  sun  and  have  a  radius  of  about  45°- 


ATMOSPHERIC  OPTICS   AND   ELECTRICITY. 


173 


These  rings  are  colored,  and  have  the  red  side  towards  the  sun.  The 
width  of  the  ring  is  about  3°,  and  although  not  so  intense  as  the  first 
kind,  yet  the  colors  are  more  distinctly  separated.  Sometimes  a  ring 
of  this  nature  is  visible  around  the  whole  sky  parallel  to  the  horizon  at 
the  altitude  of  the  sun. 


\ 


\ 


FIG.  50. 


FIG.  51. 


FIG.  52. 


3.  Rings  which  turn  a  convex  side  towards  the  sun,  and  have  a 
radius  of  about  90°.  These  are  of  weak  intensity  and  of  indefinite 
color.  Only  portions  of  the  ring  usually  are  visible ;  and  they  touch 
some  of  the  smaller  rings  which  encircle  the  sun. 

Explanation  of  Halos.  —  These  ring  phenomena  are 
mainly  due  to  the  refraction  of  the  light  by  the  ice 
crystals  or  ice  needles  of  which  the  cirrus  clouds  consist. 

These  needles  are  as- 
sumed to  be  six-sided 
(Fig.  50),  and  to  hang 
suspended  in  the  air. 
When  they  hang  verti- 
cally, and  the  rays  of 
light  pass  through  as  if 
the  needles  were  triangu- 
lar (Fig.  51),  then  the  light 
is  diffracted  as  shown  in 
Fig.  52,  and  the  ring  has 

a  radius  of  22°.  When  some  of  the  ice  needles  are  inclined  to  the  ver- 
tical, then  the  light  may  enter  at  another  surface,  and  be  diffracted 
through  90°,  and  the  ring  of  45°  will  result. 

In  order  to  cause  the  ring  of  90°  radius,  the  light  must  pass 
through  the  crystal,  as  shown  in  Fig.  53,  and  suffer  total  reflection. 


FIG. 


174  ELEMENTARY   METEOROLOGY. 

The  horizontal  ring  parallel  to  the  horizon  is  due  to  the  reflection 
of  the  light  from  the  outer  surface  of  the  ice  crystals,  which  are 
at  the  altitude  of  the  sun,  and  therefore  possess  the  proper  inclina- 
tion of  the  surfaces  to  reflect  the  sun's  rays  to  the  observer's  eye ;  and 
since  no  refraction  occurs  by  means  of  the  ice  needles,  there  is  no  color- 
ing, and  the  ring  appears  white. 

The  broken  rings  or  arcs  are  likewise  reflection  and  not  refraction 
phenomena. 

The  Glory,  Brocken  Specter,  or  Fog  Image  is  analo- 
gous to  the  coronal  phenomenon.  It  is  brought  about  by 
the  sun  casting  a  shadow  of  the  observer  on  a  fog  or  cloud 
bank.  This  shadow,  sometimes  of  huge  dimensions,  is 
surrounded  by  a  glory  of  light,  which  is  caused  by  the 
diffraction  of  the  rays  of  light  by  the  water  particles  (near 
the  observer),  and  the  resulting  separation  of  the  prismatic 
colors  which  are  reflected  to  the  eye  of  the  observer  by 
the  fog  particles. 

Sometimes,  where  the  observer  is  on  the  top  of  a  mountain,  the 
whole  peak  may  have  its  shadow  thrown  against  the  fog  bank.  At 
times  a  seemingly  distant  and  white  fog  bow  encircles  the  shadow  of 
the  observer  and  the  glory  which  surrounds  the  head.  A  glory  of 
light  is  also  sometimes  to  be  seen  around  the  shadow  of  the  head 
of  the  observer  when  it  falls  on  bedewed  or  wet  grass. 

Rainbows  are  produced  by  the  refraction  of  the  sun's 
rays  by  means  of  the  raindrops  in  the  air,  after  which  the 
separated  band  of  colors  is  then  reflected  to  the  observer's 
eye.  The  center  of  this  bow  is  opposite  to  the  sun.  The 
radius  of  the  bow  is  40°  to  42°,  with  sometimes  a  fainter 
secondary  bow  outside,  with  a  radius  of  50°  to  54°.  The 
inner  bow  has  the  blue  color  on  the  inside,  and  the  red  on 
the  outside ;  while  this  order  is  reversed  for  the  outer  bow. 
Rainbows  are  most  frequent  in  the  local  showers  in  which 
the  sun  suddenly  breaks  through  the  clouds  at  the  edge  of 


ATMOSPHERIC   OPTICS  AND   ELECTRICITY.  175 

the  storm.  Where  there  is  a  wide  distribution  of  cloudiness, 
as  in  our  long-continued  rains,  the  proper  conditions  for 
the  formation  of  the  bow  will  seldom  be  present.  Rain- 
bows are  visible  when  the  sun  is  at  low  altitudes ;  and  the 
nearer  the  latter  is  to  the  horizon,  the  greater  will  be  the 
length  of  the  bow  visible  in  the  sky.  The  bow  is  180°  in 
length,  or  a  semicircle,  when  the  sun  is  close  to  the  horizon. 

Atmospheric  Electricity We  find  the  atmospheric  air 

usually  in  a  state  of  positive  electrification ;  that  is,  it  is 
generally  charged  with  positive  electricity,  which  is  the 
condition  of  glass  when  rubbed  with  silk.  Very  great 
fluctuations,  however,  occur,  especially  during  thunder- 
storms, snowstorms,  etc. ;  and  the  electrification  becomes 
sometimes  negative,  which  is  the  condition  of  glass  when 
rubbed  with  resin  or  sealing  wax.  The  origin  of  atmos- 
pheric electricity  is  still  unaccounted  for,  but  it  may  be  due 
to  the  frictional  action  of  the  air.  The  degree  of  electrifi- 
cation of  the  air  is  measured  by  means  of  an  instrument 
called  an  electrometer. 

There  are  two  classes  of  atmospheric  electric  phenom- 
ena which  we  shall  notice :  they  are  auroral  displays, 
which  may  partly  occur  in  the  lower  atmosphere,  and 
partly  in  the  highest  layers ;  and  lightning  displays,  con- 
fined to  the  lower  air  layers. 

The  Aurora  —  called  in  the  northern  hemisphere  the 
aurora  borealis,  or  northern  lights,  and  in  the  southern 
hemisphere  aurora  australis,  or  southern  lights  —  is  an 
illumination  of  the  sky  which  occurs  in  middle  and  polar 
latitudes,  and  which  has  a  zone  of  maximum  frequency  from 
the  6oth  to  the  /oth  parallel  of  latitude  north  and  south  of 
the  equator.  It  rarely  occurs  within  the  tropics.  It  is  pos- 
sible that  some  of  the  widespread,  intensely  active,  auroral 
phenomena  have  an  electromagnetic  origin  ;  while  those  of 


1/6  ELEMENTARY   METEOROLOGY. 

local  occurrence  may  be  simply  the  electrostatic  charge  of 
the  air  layer,  rendered  luminous  by  atmospheric  changes. 

As  usually  observed  by  us,  the  aurora  consists  of  an 
arch  or  band  of  light.  It  may,  however,  have  various 
forms,  such  as  that  of  an  arch,  ribbon,  collection  of  beams, 
corona,  and  haze  or  diffused  light.  The  auroral  arch  usually 
spans  the  sky  in  an  east-westerly  direction.  In  the  middle 
latitudes  it  is  seen  to  the  north,  and  in  the  extreme  northern 
latitudes  to  the  south.  The  dimensions  are  variable.  (A 
band  observed  in  1893  was  supposed  to  be  15  miles  wide, 
perhaps  250  miles  high,  and  over  1,000  miles  long."  In  gen- 
eral, the  arch  may  be  said  to  vary  between  an  unknown  lower 
limit  and  300  miles  in  altitude.  The  arch  sometimes  varies 
in  intensity  in  different  parts.  Frequently  band-like  stream- 
ers of  light  are  seen  perpendicular  to  the  arch,  and  they 
extend  to  an  unknown  height.  Sometimes  these  stream- 
ers are  constant  during  their  existence,  and  sometimes  they 
are  intermittent  and  pulsating,  flashing  out  at  intervals. 

Aurorae  in  the  northern  hemisphere  are  least  frequent 
in  January  and  June,  and  most  frequent  in  March  and 
October.  There  seems  also  to  be  an  ii-year  period  of 
frequency  such  as  was  found  by  astronomers  for  sun  spots. 

Lightning.  —  The  air  between  two  clouds  charged  with 
electricity,  or  between  a  charged  cloud  and  the  earth,  is 
subject  to  an  electrical  strain  or  pull.  When  this  strain 
upon  the  air  column  becomes  too  great  for  further  resist- 
ance, a  disruptive  discharge  takes  place.  This  discharge 
may  vary  in  character  from  the  invisible  silent  lightning 
to  the  violent  impulsive  rush  discharge  which  has  an 
enormous  amount  of  energy.  The  lightning  flash  is  the 
luminously  heated  air  in  the  path  of  the  discharge.  The 
flash  may  have  a  duration  varying  from  3-^  of  a  second  to 
a  second.  The  thunder  is  but  the  crackle  of  the  electric 


ATMOSPHERIC   OPTICS   AND    ELECTRICITY.  177 

discharge,  but  its  sound  is  frequently  reenforced  and  pro- 
longed by  numerous  refractions  and  reflections  from  adja- 
cent clouds  and  other  objects. 

The  needle  of  an  electrometer  which  indicates  the  amount  of  the  elec- 
trical strain  will  show  by  its  fluctuations  or  "  breaks  "  not  only  all  of  the 
flashes  or  discharges  of  lightning  which  are  visible,  but  also  the  silent 
discharges  which  are  of  frequent  occurrence  but  are  invisible. 

The  typical  forms  of  lightning  flashes  (which  have  been  studied  by 
the  aid  of  photography)  are,  — 

1 .  Stream   lightning,   which   consists   of  a   plain,    broad,    smooth 
streak  or  flash  of  light. 

2.  Sinuous  lightning,  which  consists  of  a  flash  following  some  one 
general  direction ;  but  the  line  is  sinuous,  bending  from  side  to  side. 
This  is  the  most  common  type. 

3.  Ramified  lightning,  in  which  part  of  the  flash  appears  to  branch 
off  from  the  main  stem  like  the  branches  of  a  tree  from  the  trunk ;  but 
whether  these  branches  issue  from  the  trunk  or  unite  with  it,  is  un- 
known. 

4.  Meandering  lightning,  in  which   the   flash   appears   to  wander 
about  without  any  definite  course  and  form  irregular  loops. 

5.  Beaded  lightning,  in  which  a  series  of  bright  beads  of  light  appear 
along  the  white  streak  of  lightning. 

6.  Dark  flashes  of  lightning  have  also  been  photographed. 
The  causes  of  the  different  forms  are  not  well  understood. 

Since  the  electrical  discharge  between  the  clouds  and 
the  ground  will  take  place  more  readily  where  the  distance 
is  least  (other  conditions  being  equal),  it  is  quite  probable 
that  the  points  of  the  clouds  which  appear  suspended  from 
the  main  thundercloud  mass  will  the  most  frequently  form 
the  places  from  which  the  discharge  takes  place.  In  case 
of  a  tornado-funnel  cloud,  in  which  the  cloud  reaches  nearly 
or  quite  to  the  earth,  the  condition  for  a  discharge  along 
this  would  be  most  favorable.  Also  at  the  time  of  the 
greatest  rainfall  the  electric  discharge  would  be  greatly 
facilitated  by  the  excellence  of  the  conduction  which  would 


ELEMENTARY    METEOROLOGY. 


be  established  by  the  falling 
water  between  the  cloud  and 
the  earth. 

Periodic  Changes  in  Atmos- 
pheric Electricity.  —  There  is  a 
periodic  daily  change  in  the  de- 
gree of  electrification  of  the  air. 
There  are  times  of  maxima  at 
about  eight  or  nine  o'clock  in  the 
1  morning  and  evening,  and  of 

5  minima  at  two  to  four  o'clock  at 
s 

night  and  in  the  afternoon.  The 
cause  of  this  periodicity  is  not 
known  with  certainty. 

There  is  an  irregular  annual 
periodic  change,  of  which  we 
can  only  say  that  in  general  the 
maximum  electrification  occurs 
in  the  cold  season,  and  the 
minimum  in  the  warm  season. 

The  accompanying  diagram  (Fig. 
54)  shows  the  fluctuations  in  the 
*?  potential  of  atmospheric  electricity 
during  a  couple  of  days,  as  observed 
by  means  of  an  electrometer.  In  the 
curve  not  only  do  we  find  rapid  fluctua- 
tions in  the  positive  electrification,  but 
there  are  also  sudden  changes  from  a 
powerful  positive  to  an  almost  equally 
powerful  negative  electrification  when 
the  thunderstorm  is  in  progress. 

St.  Elmo's  Fire  is  an  elec- 
trical phenomenon  sometimes 


ATMOSPHERIC  OPTICS   AND   ELECTRICITY  179 

seen  as  a  continuous  luminous  discharge  from  elevated 
points.  It  usually  has  the  appearance  of  jets  of  flame 
issuing  from  the  points  of  trie  objects,  and  is  accompanied 
by  a  hissing  or  buzzing  noise.  It  is  most  frequent  in 
winter,  and  is  almost  invariably  accompanied  by  a  heavy 
fall  of  soft  hail  or  snow. 


CHAPTER   VIII 
GENERAL  CIRCULATION  OF  THE   ATMOSPHERE 

Some  Preliminary  Ideas  concerning  Air  Motions, — We 

come  now  to  the  study  of  the  systems  of  movements  of 
the  air,  which  have  already  been  mentioned  in  the  chapter 
on  winds.  It  is  first  necessary,  however,  to  state  in  a  gen- 
eral way  the  nature  of  air  motions,  and  the  changes  which 
air  undergoes,  and  its  conditions  with  respect  to  the  sur- 
rounding air,  when  subjected  to  these  motions,  especially 
to  vertical  motions.  These  are  briefly  stated  in  a  neces- 
sarily disconnected  manner,  and  their  bearing  or  impor- 
tance cannot  be  fully  seen  until  they  are  applied  to  proper 
cases  as  they  come  up  in  their  natural  order.  . 

General  Air  Motions.  —  Atmospheric  motions  may  in 
general  be  divided,  according  to  direction,  into  two  classes, 
—  horizontal  and  vertical.  Air  currents  are  seldom  in- 
clined at  a  large  angle  (such  as  20°  or  45°)  to  the  hori- 
zontal or  vertical  directions. 

Horizontal  air  motions  are  those  nearly  parallel  to  the 
earth's  surface,  and  are  produced  by  gradient  forces  effec- 
tive in  those  directions.  In  horizontal  currents  the  changes 
in  density  and  temperature  of  the  moved  air  mass  are  usu- 
ally very  gradual  and  relatively  slight. 

Vertical  air  motions  arise  from  two  causes.  They  are 
either  a  natural  consequence  of  the  horizontal  motions,  in 
order  to  preserve  a  continuity  of  the  air  taking  part  in  these 
motions  when  they  have  dissimilar  directions ;  or  they  are 

180 


GENERAL  CIRCULATION   OF  THE   ATMOSPHERE.  i8l 

due  to  the  endeavor  of  a  local  mass  of  air  having  an 
abnormal  condition  to  move  to  a  place  where  its  condi- 
tion will  be  normal,  i.e.,  the  same  as  that  of  the  mass 
of  surrounding  air.  In  vertical  currents  the  changes  in 
density  and  temperature  of  the  moved  air  are  rapid  and 
of  relatively  great  magnitude. 

Centrifugal  Force.  —  If  a  body  (a  mass  of  air,  for  in- 
stance) has  a  motion  in  any  direction,  it  will  continue  to 
move  in  a  straight  line  with  constant  velocity,  unless  it  is 
acted  upon  by  some  outside  force  or  is  subjected  to  some 
sort  of  resistance.  When  a  body  is  constrained  to  move 
in  a  circular  path,  it  still  has  a  tendency  to  move  in  a 
straight  line,  which  would  be  in  the  direction  of  the  tan- 
gent to  its  circular  path,  at  whatever  point  the  body  hap- 
pens to  be.  The  force  with  which  the  body  presses  out- 
ward, in  its  endeavors  to  depart  from  the  circular  motion,  is 
called  the  centrifugal  force.  This  force  would  be  the  pull 
exerted  on  a  cord  by  a  weight  attached  to  one  end  of  the 
cord,  if  it  were  whirled  around  in  a  circle  of  which  the 
other  end  of  the  cord  (or  any  part  of  it)  was  the  fixed 
center  of  revolution. 

In  case  a  mass  of  free  air  is  forced  to  move  in  a  curvi- 
linear path,  it  will  exert  a  certain  amount  of  centrifugal 
force  outward  from  the  center  of  rotation  ;  and  the  amount 
of  this  force  is  exerted  as  a  pressure  against  outer  adjacent 
masses  of  air.  Since  air  is  an  easily  yielding  and  com- 
pressible substance,  the  effect  of  this  centrifugal  force  is 
to  diminish  the  amount  of  air  within  the  circle,  and  to 
increase  the  amount  without  the  circle.  Where  the  whole 
of  a  mass  of  air  has  a  rotation  around  a  point  at  its  center, 
the  centrifugal  force  causes  the  air  to  recede  most  at  this 
center,  and  then  less  and  less  at  increasing  distances  from 
the  center. 

WALDO    METEOR. —  II 


1 82  ELEMENTARY  METEOROLOGY. 

Conservation  of  Areas.  —  When  a  body  (air,  for  example) 
is  acted  on  by  a  central  force,  or  the  opposite,  and  has  a 
gyratory  motion  around  this  center,  then  the  varying  line 
connecting  this  body  with  the  center  (called  radius  vector) 
sweeps  over  equal  areas  of  space  in  equal  times.  There- 
fore when  the  air  approaches  the  center  of  revolution,  its 
velocity  of  revolution  must  increase  as  the  radius  vector 
shortens  ;  and  conversely,  when  it  recedes  from  the  center, 
the  velocities  decrease. 

Energy  of  the  Air.  —  While,  from  the  fact  that  we  live 
in  the  lower  part  of  the  atmosphere,  we  must  deal  mostly 
with  the  winds  near  the  surface  of  the  ground,  yet  it  must 
not  be  assumed  that  the  upper  half  of  the  air  (that  is,  the 
portion  above  the  altitude  of  about  three  miles)  can  be 
neglected  in  studying  its  conditions,  for  it  has  been  com- 
puted that  in  this  upper  half  of  the  atmosphere  there  is 
six  times  as  much  gradient  force  or  energy  productive  of 
motion  as  in  the  lower  half.  The  altitude  of  about  40,000 
feet  above  sea  level,  which  is  about  the  altitude  of  the  high- 
est clouds  (the  cirrus  clouds),  divides  the  energy  of  the  at- 
mosphere into  two  parts.  The  amount  of  gradient  force 
or  energy  above  this  altitude  is  equal  to  that  below  it. 

The  Heat  in  the  Air.  —  The  heat  which  is  in  the  atmos- 
phere is,  as  we  have  seen,  derived  by  a  number  of  proc- 
esses. There  is  first  absorbed  about  half  of  the  direct 
solar  heat ;  then  about  half  of  the  heat  reflected  from  the 
ground  and  water  surface  of  the  earth  ;  then  all  the  heat 
supplied  from  the  earth's  surface  by  convective  currents, 
and  the  heat  supplied  by  radiation  from  the  earth's  surface 
(just  as  it  is  radiated  from  the  sun).  All  this  heat  must 
be  lost  from  the  air  through  a  process  of  radiation  from 
the  air  itself,  which  must  not  be  confused  with  the  radiation 
through  the  air  from  other  objects. 


GENERAL   CIRCULATION   OF  THE   ATMOSPHERE  183 

Radiation  of  Heat  from  Air.  —  The  process  of  radiation 
of  heat  from,  and  of  absorption  of  heat  by,  layers  of  air 
within  the  atmosphere,  depends  on  the  difference  of  tem- 
perature, and  perhaps  on  the  difference  of  density,  of  these 
layers ;  and  without  doubt  a  considerable  portion  of  the 
heat  which  escapes  from  lower  air  layers  passes  directly 
through  the  upper  air  into  space,  and  the  portion  of  heat 
which  is  absorbed  by  the  air  passes  in  its  turn  through 
the  layers  still  above,  and  finally  makes  its  escape. 

This  radiation  of  heat  from  the  air  itself  is  shown  by 
the  fact  that  the  cooling  of  the  air  which  takes  place  on  a 
clear  night  is  quite  general;  while,  if  it  depended  on  the 
conduction  of  the  heat  from  the  air  to  the  cooler  ground 
to  replace  that  lost  by  radiation  from  the  latter,  there 
would  be  a  cooling  of  the  air  to  a  height  of  not  more 
than  10  feet  during,  say,  twelve  hours  of  darkness. 

The  rate  of  actual  cooling  of  the  air  by  radiation  varies 
approximately  as  the  difference  in  temperature  of  the  air 
and  its  surroundings,  and  it  is  therefore  probably  least  at 
the  middle  altitudes  of  the  atmosphere. 

Comparison  of  Solar  Heat  directly  absorbed  by,  and 
radiated  from,  the  Air.  —  The  amounts  of  heat  absorbed  by 
the  air  and  radiated  from  it  again  are  nearly  equal,  under 
the  maximum  conditions  of  absorption,  during  the  same 
period  of  time.  But  since  the  radiation  from  the  air  into 
space  goes  on  at  all  times,  and  since  the  direct  absorption 
of  the  solar  heat  rays  can  take  place  only  when  the  sun  is 
shining,  the  outward  radiation  is  much  more  effective  in 
cooling  the  air  than  is  the  direct  action  of  the  sun's  rays 
in  warming  it. 

Adiabatic  Heating  and  Cooling  of  Air  Dynamically.  - 
The  adiabatic   heating  and  cooling  of  masses  of  air  by 
the  variation  of  the  air  pressure,  due  to  the  ascending  or 


184  ELEMENTARY   METEOROLOGY. 

descending  movement  of  the  air,  is  one  of  the  most  impor= 
tant  factors  in  atmospheric  events. 

Cooling  of  Dry  Air  in  Ascending,  and  Heating  in  Descend- 
ing Currents.  —  Theoretically,  the  rate  of  cooling  in  ascend- 
ing and  the  rate  of  warming  in  descending  dry  air  is  i°  F. 
for  each  183  feet  change  in  altitude,  whatever  the  temper- 
ature of  the  air.  If  the  air  is  in  any  way  subjected  to  a 
change  of  pressure  equivalent  to  that  which  would  occur 
in  passing  through  183  feet  in  altitude,  it  will  experience 
the  same  changes  of  temperature  as  it  would  if  actually 
moved  through  this  distance. 

Action  of  Ascending  Currents  of  Moist  Air»  —  We  have 
just  been  considering  the  air  when  it  is  dry  ;  but  when 
moisture  is  present,  as  it  always  is  in  the  atmosphere, 
we  have  no  longer  the  simple  conditions  just  mentioned. 
The  air,  as  it  cools  adiabatically  in  its'  ascent  (as  just 
discussed),  has  a  diminished  capacity  for  moisture ;  and 
when  the  limiting  temperature  is  reached,  at  which  con- 
densation of  the  moisture  present  occurs,  then,  with  the 
condensation  which  follows,  there  is  a  freeing  of  latent 
heat,  which  retards  the  further  cooling  of  the  air.  Thus 
there  is  heat  added  to  the  air  mass ;  and  if  the  air  keeps 
on  in  its  ascent,  the  decrease  of  i°  F.  per  183  feet  of 
ascent  is  diminished  by  the  amount  of  latent  heat  supplied 
by  this  condensation.  But  until  condensation  does  actually 
take  place,  the  rate  of  this  decrease  of  temperature  is  nearly 
i°  F.  per  183  feet,  even  when  moisture  is  in  the  air. 

Descending  Currents  of  Moist  Air.  —  The  rate  at  which 
air  becomes  heated  in  descending  is  i°  F.  for  each  183 
feet,  whether  the  air  is  dry  or  saturated  (except  where 
rain  is  held  in  the  air,  or  where  supersaturation  exists). 
Thus  the  rate  of  change  of  temperature  may  be  very  dif- 
ferent in  ascending  and  descending  currents  of  moist  air. 


GENERAL   CIRCULATION   OF  THE   ATMOSPHERE.  185 

The  Actual  Rate  of  Decrease  of  Temperature  with  the  alti- 
tude is  less  than  that  assigned  by  theory,  and  varies  some- 
what on  account  of  various  irregularities  in  the  existing 
atmospheric  conditions.  It  is  greater  in  summer  than  in 
winter,  and  differs  in  cloudy  weather  from  that  in  clear 
weather.  Still  these  irregularities  are  but  departures  from 
the  laws  just  mentioned,  and  their  causes  would  have  to  be 
examined  separately. 

Indifferent  Equilibrium.  —  If  the  temperature  of  the 
unsaturated  quiescent  or  horizontally  moving  air  adjacent 
to  or  surrounding  a  vertical  current  of  air  decreases  i°  F. 
for  each  183  feet  increase  in  altitude,  then  the  air  is  said  to 
be  in  indifferent  equilibrium.  For  unsatu-rated  air  which 
is  moved  upward  or  downward,  and  which  changes  its  tem- 
perature at  this  rate,  will  remain  in  the  new  position  in 
which  it  is  placed,  because  its  change  of  temperature  has 
been  just  such  as  to  accommodate  itself  to  the  temperature 
of  the  surrounding  air  in  its  new  position. 

Stable  Equilibrium.  —  Unsaturated  quiescent  air  which 
decreases  in  temperature  at  a  rate  less  than  i°  F.  for  each 
183  feet  of  increase  in  elevation  is  said  to  be  in  stable 
equilibrium :  for  when  a  portion  of  such  air  receives  an 
upward  motion,  its  density  'gradually  becomes  greater  than 
that  of  the  surrounding  air  at  the  same  level,  and  it  would 
sink  back  again  to  its  starting  place  if  the  force  which 
moved  it  ceased  to  act. 


The  differences  in  air  density  depend  on  the  relation  of  decrease  of 
temperature  with  altitude,  in  the  air  which  is  in  motion,  to  that  in  the 
air  which  surrounds  it.  Since  the  rate  of  decrease  of  temperature  of 
the  unsaturated  ascending  air  is  about  i°  F.  for  each  183  feet  of  ascent, 
then,  if  the  temperature  of  the  surrounding  quiet  air  decreases  at  a  rate 
less  than  this,  the  ascending  air  will  cool  the  more  rapidly,  and  will 
become  the  denser  as  it  ascends ;  and  when  the  force  which  made  it 


1 86 


ELEMENTARY   METEOROLOGY. 


ascend  is  spent,  the  air  which  has  ascended  will  fall  back  again  to  its  orig- 
inal position.  Similarly,  if  a  downward  current  of  air  becomes  heated 
faster  than  the  surrounding  air,  then  it  becomes  lighter  than  the  air  at 
any  lower  altitude  at  which  it  may  arrive ;  and  when  the  force  which 
causes  it  to  descend  is  spent,  the  air  will  rise  again  to  its  former  position. 

Unstable  Equilibrium.  —  Unsaturated  quiescent  air  which 
decreases  in  temperature  at  a  rate  greater  than  i°  F.  for  each 
183  feet  of  increase  in  elevation  is  said  to  be  in  imstable 
equilibrium:  for  when  a  portion  of  such  air  is  moved  up- 
wards it  becomes  lighter  than  the  surrounding  air,  and 
when  moved  downwards  it  becomes  still  heavier  than  the 
surrounding  air,  at  any  level  which  it  may  reach ;  and  thus 
there  is  a  tendency  for  the  moving  air  to  continue  in  the 
direction  in  which  it  is  started. 

This  condition  occurs  when  the  temperature  of  the  moving  air  varies 
at  a  rate  less  than  that  of  the  surrounding  quiescent  air. 

Conditions  of  Equilibrium  illustrated  numerically.  —  In  the  accom- 
panying table  (from  Ferrers  "  Winds  "),  the  first  column  gives  the  alti- 
tudes ;  the  second  column,  an  assumed  temperature  at  these  altitudes 
which  would  produce  stable  equilibrium ;  the  third  column,  the  tem- 
peratures which  would  produce  indifferent  equilibrium ;  and  the  fourth 
column,  an  assumed  temperature  which  would  produce  unstable  equi- 
librium in  case  vertical  currents  arise. 


ALTITUDE. 

TEMPERATURES  FOR 
STABLE  EQUILIBRIUM. 

TEMPERATURES  FOR 
INDIF.  EQUILIBRIUM. 

TEMPERATURES  FOR 
UNSTABLE  EQUILIBRIUM. 

Feet. 

F. 

F. 

F. 

9,840 

42.8° 

32° 

21.2° 

8,200 

50.0 

41 

32-0 

6,560 

57-2 

50 

42.8 

4,920 

64.4 

59 

53-6 

3,280 

71.6 

68 

64.4 

1,640 

78.8 

77 

75-2 

ooo 

86.0 

86 

86.0 

GENERAL  CIRCULATION   OF  THE  ATMOSPHERE.  187 

Here  we  see,  that  since  a  vertical  air  current  has  a  change  of  tem- 
perature with  the  altitude  of  about  i°  F.  per  183  feet,  then,  when  the 
rate  of  change  of  temperature  in  the  surrounding  quiescent  air  is  less 
than  this,  the  condition  is  that  of  stable  equilibrium ;  when  it  is  equal 
to  it,  the  condition  is  that  of  indifferent  equilibrium ;  and  when  it  is 
greater,  the  condition  is  that  of  unstable  equilibrium. 

General  Circulation  of  the  Atmosphere.  —  The  winds  be- 
longing to  the  general  circulation  of  the  atmosphere  are  the 
hemispherical  systems  of  winds  extending  from  the  equa- 
tor to  the  poles.  It  has  been  found  by  observations  of 
wind  vanes,  that  in  the  northern  hemisphere,  in  the  lower 
air,  there  is  a  decided  prevailing  wind  blowing  from  the 
east  or  northeast  over  the  region  extending  from  near  the 
equator  almost  to  the  tropics ;  while  from  a  little  beyond 
the  tropics  to  the  polar  regions  there  is  a  prevailing  wind 
from  the  west  or  southwest.  Directly  at  the  equator  and 
in  the  region  of  the  tropics,  there  is  no  such  prevailing 
direction  of  wind,  and  in  fact  these  may  be  designated 
regions  of  relative  calms.  These  surface  wind  directions 
are  shown  in  a  general  way  by  the  arrows  (drawn  flying 
with  the  wind)  on  the  charts,  Figs.  26  and  27,  and  are  also 
shown  by  the  heavy  arrows  on  the  inner,  shaded  portion 
of  Fig.  55. 

At  high  altitudes,  mostly  in  the  regions  above  the  highest  clouds, 
there  are  prevailing  winds  blowing  from  the  west  or  southwest  over 
the  entire  hemisphere  from  the  equatorial  to  the  polar  regions. 

Between  the  winds  of  the  upper  and  lower  regions,  in  which  the 
wind  direction  is  mostly  poleward,  and  extending  to  the  ground  in  the 
polar  regions,  there  exist  return  currents  flowing  equatorward^  which 
prevent  the  accumulation  of  air  in  the  polar  regions. 

It  must  not  be  supposed  that  these  general  wind  direc- 
tions are  continuous  and  uninterrupted.  As  a  matter  of 
fact,  there  are  continually  occurring  within  them  local  air 


i88 


ELEMENTARY   METEOROLOGY, 


movements  of  sometimes  thousands  of  miles  lateral  extent, 
which  entirely  change,  during  their  continuance,  the  direc- 
tion of  the  main  air  currents.  So  it  is  only  by  observing 
the  winds  for  a  great  length  of  time  that  the  average  nor- 
mal direction  can  be  determined. 

In  the  soutJiern  hemisphere  these  wind  directions  are 
somewhat  reversed  :  the  prevailing  lower  currents  between 
the  Tropic  of  Capricorn  and  the  equator  are  from  the 


FIG.  55. —DIRECTIONS  OF  PRIMARY  AIR  CURRENTS  (AFTER  FERREL). 

southeast,  and  those  to  the  poleward  of  the  tropic  are 
from  the  northwest. 

The  upper  air  has  a  general  movement  from  the  west 
and  northwest,  while  for  middle  altitudes  the  movement 
is  from  the  pole  equatorward.  In  the  polar  regions  this 
counter  or  return  current  reaches  to  the  ground. 

Schematic  Diagram  of  the  General  Atmospheric  Circula- 
tion.— The  accompanying  diagram  (Fig.  55)  shows  the 
main  features  of  the  general  atmospheric  circulation.  On 


GENERAL  CIRCULATION   OF  THE  ATMOSPHERE.  189 

the  shaded  sphere,  the  complete  arrows  show  the  direction 
of  the  lower  air  currents,  and  the  broken  arrows  that  of 
the  upper  currents ;  the  arrows  flying  with  the  wind. 

The  outer,  unshaded  portion  of  the  figure  shows  the 
circulation  projected  into  the  plane  of  the  meridian;  and 
the  position  of  an  isobaric  surface  at  low  altitudes  is  rep- 
resented by  the  inner,  and  that  of  an  isobaric  surface  at 
high  altitudes  by  the  outer,  encircling  line. 

On  the  body  of  the  sphere  we  see  the  eastward  upper 
current  at  all  latitudes,  and  the  lower  current  eastward  in 
the  higher  latitudes,  but  westward  in  low  latitudes. 

The  causes  of  these  wind  movements,  and  the  connec- 
tion between  them,  have  been  studied  out  with  great  care ; 
and  some  of  their  details  are  now  to  be  entered  upon,  after 
comprehending  which,  the  general  air  motions  shown  in 
Fig.  55  can  be  more  readily  understood. 

The  Cause  of  the  General  Circulation  of  the  Atmos- 
phere is  primarily  the  large  but  not  constant  difference 
in  temperature  existing  between  the  equatorial  and  polar 
regions.  These  atmospheric  motions  embrace,  generally 
speaking,  a  whole  hemisphere  each  side  of  the  equator. 
The  difference  in  the  air  temperature  between  the  poles 
and  the  equator  amounts  (at  a  few  feet  above  the  ground) 
to  about  80°  F.  for  the  average  of  the  entire  year.  Since 
the  colder  the  air  the  denser  it  is,  any  isobaric  surface  is 
nearer  the  ground  at  the  cold  pole  than  it  is  at  the  warm 
equator.  There  exists,  then,  a  declination  of  isobaric  sur- 
faces and  barometric  gradients  in  the  direction  of  the 
poles,  and  there  is  a  movement  of  the  air  down  these 
isobaric  surfaces  after  the  manner  already  described.  This 
movement  down  the  decline  of  the  isobaric  surfaces  from 
the  equator  towards  the  pole  continues  until  the  alti- 
tude of  these  isobaric  surfaces  becomes  so  lessened  at  the 


ELEMENTARY   METEOROLOGY. 


A 


equator  and  increased  at  the  pole  as  to  bring  them  to  the 
same  level  throughout.  This  motion  originally  occurs  at 
all  altitudes  where  the  isobaric  surfaces  decline  towards 
the  pole  ;  but  it  increases  with  the  altitude,  because  the 

difference  between  any 
two  upper  air  layers 
must  be  added  to  the 
differences  which  al- 
ready existed  for  the 
layers  below.  In  Fig. 
56,  D  is  the  equator, 
CC  are  the  poles,  and 

BAB  an  isobaric  surface.  The  arrows  show  the  direction 
of  the  flow  of  air. 

As  a  result  of  these  air  currents   towards  the  poles,  there  arise 
(as   is  explained   later)  other  nearly  horizontal  currents  towards  the 
equator,  and  upward  and  downward  currents   connecting  them  (see 
Fig-  57)-     It  has  been  com- 
puted   that    if    the    average  ^_  ^ 
temperature   of  an  air   layer 
33,000   feet   in  thickness,  at 
latitude  30°,  were  40°  F.,  and 
if  the  temperature  at  the  pole 
were  90°  F.   less   than  this, 
then  the  upper  isobaric  sur- 
face of  this  layer  would  be 
depressed    so    much   that   it 
would  be  nearly  6,000  feet  lower  at  the  pole  than  at  latitude  30° ;  and  a 
mass  of  air  gliding  down  this  inclined  surface  would  attain  at  the  pole 
a  velocity  of  over  600  feet  per  second,  if  it  started  at  latitude  30°  from 
a  position  of  rest,  and  its  motion  were  frictionless  and  undisturbed, 
and  under  the  influence  of  gravity  alone. 

At  the  earth's  surface  the  observed  greatest  pressure  is  at  about  lati- 
tude 30°,  and  the  calculated  depression  of  the  isobaric  surface  is  260 
feet  at  the  equator,  and  in  the  southern  hemisphere  at  the  pole  it  is 
flbout  650  feet ;  while  in  the  northern  hemisphere  the  maximum  de- 


FIG.  57. — IDEAL  MERIDIONAL  AIR  CIRCULATION. 


GENERAL  CIRCULATION   OF  THE  ATMOSPHERE,  191 

pression  is  only  about  325  feet,  and  is  at  latitude  60° ;  from  thence  to 
the  north  pole  the  depression  lessens. 

On  the  basis  of  observed  air  pressures  at  sea  level,  it  has  been  com- 
puted that  at  an  altitude  of  33,000  feet  the  depression  of  the  isobaric 
surface  amounts  to  over  2,800  feet  at  the  north  pole,  and  3,150  feet 
at  the  south  pole. 

Diurnal  Rotation  of  the  Earth.  —  In  addition  to  the  baro- 
metric gradient  due  to  the  difference  in  the  temperature  at 
the  equator  and  the  pole,  there  is  another  influence  power- 
fully affecting  the  general  air  motions,  and  this  is  the 
diurnal  rotation  of  the  earth  on  its  axis.  In  considering 
the  matter  of  air  motions,  however,  it  is  best  first  to 
examine  the  conditions  resulting  primarily  from  the  differ- 
ences of  temperature  alone,  and  then  to  add  the  modifying 
influences  of  the  earth's  rotation. 

Ideal  General  Circulation  of  the  Air  without  Rotation  of 
the  Earth.  —  If  the  motion  of  rotation  of  the  earth  on  its 
axis  is  not  considered,  then  the  movement  of  a  particle  of 
air  is  either  north  and  south,  or  upward  and  downward, 
in  the  plane  of  its  meridian. 

Starting  with  the  high  temperature  at  the  equator,  and 
the  low  temperature  at  the  pole,  and  the  resulting  inclina- 
tion of  the  isobaric  surfaces  from  the  equator  towards  the 
pole,  we  find  the  equatorial  air  flowing  poleward  down  these 
surfaces  in  the  direction  of  the  lowest  level  with  a  velocity 
which  increases  with  the  altitude.  The  upper  air,  then, 
flows  poleward  most  rapidly ;  but  at  the  earth's  surface 
there  is  no  motion,  because  at  first  there  is  no  gradient  at 
this  lowest  surface,  the  amount  of  air  above  being  every- 
where the  same.  Just  as  soon,  however,  as  any  of  the 
upper  air  leaves  its  position  in  lower  latitudes,  there 
is  a  lessening  of  the  actual  weight  of  the  air  (pressure 
at  the  surface  of  the  earth)  there,  and  an  increase 


192  ELEMENTARY   METEOROLOGY. 

towards  the  pole,  by  reason  of  the  air  which  has  flowed 
thence;  and  a  countercurrent  of  air 'sets  in  along  or  near 
the  surface  of  the  earth  from  the  region  of  the  pole 
towards  the  equator,  to  take  the  place  of  the  air  which 
moves  away  above  from  the  latter  region.  We  thus  have 
a  current  of  air  above  flowing  poleward,  and  a  current 
below  flowing  equatorward ;  and  in  order  to  satisfy  the 
conditions  of  continuity,  which  allows  no  gaps  to  occur  in 
flowing  fluids,  there  must  exist  vertical  currents  at  the 
ends  of  these  two  horizontal  currents,  in  order  to  complete 
the  circuit  by  connecting  them. 

The  direction  of  the  upward  and  downward  connecting 
currents  is  conditioned  by  the  direction  of  flow  of  the 
main  horizontal  currents,  for  they  must  be  such  as  to  keep 
up  a  connected  flow  of  air.  Thus  an  upward  current  at 
the  equator  and  a  downward  current  at  the  pole  are 
needed  to  meet  this  requirement.  The  circuit  is  shown 
in  the  diagram  (Fig.  57). 

Since  the  temperature  differences  at  the  poles  and  the 
equator  are  perpetual,  and  the  force  (gravity)  which  causes 
the  poleward  motion  is  a  continuous  one,  this  circuit  of 
the  air  current  is  uninterrupted  in  action  after  it  once 
begins.  And,  moreover,  since  the  cause  of  the  motion 
acts  continuously,  there  would  be  a  continually  increasing 
velocity  in  this  circuit,  if  it  were  not  for  the  loss  of  motion 
through  friction,  and  the  interference  or  mixing  of  air 
masses  having  different  directions  and  velocities  of  motion. 

Regions  of  Separation  between  the  Horizontal  and  Verti- 
cal Currents,  respectively.  —  It  is  evident  that  there  must 
be  some  region  between  the  upper  poleward  air  current 
and  the  lower  current  towards  the  equator,  in  which  there 
is  no  horizontal  motion  one  way  or  the  other ;  and  likewise 
between  the  upward  and  downward  currents  at  the  equator 


GENERAL   CIRCULATION  OF  THE   ATMOSPHERE.  193 

and  the  pole  there  must  be  a  region  where  there  is  no 
vertical  motion,  and  where  the  motion  changes  from  one 
direction  to  the  other.  These  regions  of  no  motion  are 
called  neutral  planes  (see  Fig.  58). 

While  at  the  neutral  plane  between  the  upper  and  lower 
air  currents  there  is  no  horizontal  motion  in  either  direc- 
tion, yet,  with  increase  in  altitude  above  this  plane  there 
is  a  gradual  increase  in  the  poleward  velocities,  and  below 
this  plane  there  is  an  increase  in  the  equatorward  veloci- 
ties down  to  within  a  few  hundred  feet  of  the  ground, 
where  the  friction  of  the  earth  retards  the  air  motion,  and 
makes  it  decrease  again  with  nearer  approach  to  the 


Equator 
TIG.  58.  — REGIONS  OF  No  AIR  MOTIONS. 

ground.  Likewise  at  the  neutral  plane  between  the  verti- 
cal air  currents  at  the  pole  and  the  equator,  there  is  an 
increase  of  the  downward  motion  towards  the  pole,  and 
an  increase  of  the  upward  motion  towards  the  equator. 
At  the  intersection  of  these  neutral  planes  there  is  no 
motion  in  any  direction,  and  consequently  a  calm. 

Effect  of  the  Convergence  of  Meridians.  —  We  have  been 
picturing  the  motion  along  the  meridian.  Now,  it  is  neces- 
sary to  take  into  account  the  fact  that  the  meridians  con- 
verge more  and  more  as  the  equator  is  departed  from,  and 
meet  in  a  point  at  the  pole.  The  effect  of  this  conver- 
gence is  to  gradually  decrease  (with  the  decreasing  size  of 
the  parallels  of  latitude)  the  amount  of  space  (between  the 
different  meridians)  which  can  be  occupied  by  the  air  above, 


194  ELEMENTARY    METEOROLOGY. 

which  flows  poleward ;  and  to  gradually  increase  (with  the 
increasing  size  of  the  parallels  of  latitude)  the  amount  of 
available  space  which  can  be  occupied  by  the  air  below, 
which  is  flowing  from  the  polar  regions  equatorward. 
Since,  with  assumed  constant  velocity  of  motion,  about 
half  of  the  air  must  be  to  the  poleward,  and  half  to  the 
equatorward,  of  the  vertical  neutral  plane,  the  effect  of 

the  convergence  of  the 
meridians  is  to  force  the 
vertical  neutral  plane 
equatorward ;  and  it 
must  lie  somewhere 
about  latitude  30°,  be- 
cause  this  divides  the 
surface  of  a  hemisphere 
f )  (and  consequently  the 
cubic  space  between 
the  earth's  surface  and 

FIG.   59. —  REGIONS  OF  AIR   MOTIONS  AND  CALMS   the    OUter    limit    of    the 

ALONG  A  MERIDIAN.  air)into  two  equal  parts. 

This  affects  both  the  velocities  and  paths  of  moving  air 
masses. 

Meridional  Movement  of  an  Air  Mass.  —  If  we  follow  the 
motion  of  a  mass  of  air  starting  out  from  a  position  near 
the  equator  but  at  a  considerable  altitude  above  the  earth's 
surface,  and  above  the  level  of  the  horizontal  neutral  plane 
(Fig.  59),  we  shall  find  that  the  horizontal  motion  of  the  air 
mass  as  it  moves  towards  the  pole  is  accelerated  for  a  time 
until  it  reaches  some  intermediate  latitude,  and  then  it  is 
retarded  as  it  approaches  its  polar  limit ;  and  with  this  re- 
tardation there  is  a  gradual  downward  fall  of  the  air  until 
it  reaches  the  point  where  it  is  nearest  the  pole,  which 
occurs  at  the  altitude  of  the  horizontal  neutral  plane. 


GENERAL  CIRCULATION   OF  THE  ATMOSPHERE.  195 

The  air  ma&s  now  commences  its  return  journey  in  the 
lower  current  towards  the  equator :  there  is  at  first  a  con- 
tinued downward  motion,  and  then  it  becomes  more  nearly 
horizontal  and  continues  so  until  after  the  neutral  vertical 
plane  is  passed;  there  is  an  acceleration  of  velocity  until  an 
intermediate  latitude  is  reached,  after  which  the  horizontal 
motion  is  retarded,  and  there  is  a  gradual  rise  of  the  air 
mass  until  it  reaches  its  place  of  starting. 

The  circuit  made  by  the  air  masses  is  somewhat  elliptical 
(oval)  in  form ;  and  it  is  seen  that  some  of  the  air  masses 
move  in  large  orbits  which  extend  from  the  equator  almost 
to  the  pole,  while  others  move  in  smaller  orbits,  some  of 
which  extend  only  a  short  distance  each  side  of  the  inter- 
section of  the  two  neutral  planes.  All  these  motions  are 
along  the  meridians. 

The  horizontal  current  reaches  its  greatest  velocity  in  middle  lati- 
tudes, and  vanishes  at  the  equator  and  the  poles.  The  vertical  current 
disappears  at  the  earth's  surface  and  at  the  outer  limits  of  the  atmos- 
phere. The  descending  velocity  near  the  pole  is  less  than  the  ascending 
velocity  near  the  equator,  because  the  downward  current  meets  with  more 
resistance  than  the  upward  current.  The  downward-moving  air  meets 
air  of  increasing  density,  and  the  surface  of  the  earth  offers  resistance  to 
its  motion.  The  upward  current  carries  the  air  into  masses  of  less  den- 
sity, and  there  is  no  resistance  from  a  rigid  body  such  as  the  earth's 
surface.  The  velocities  of  the  vertical  currents  are  relatively  small,  but 
their  great  meridional  extent  magnifies  the  importance  of  their  action. 
The  relation  of  the  horizontal  flow  of  air  to  the  vertical  is,  roughly 
speaking,  as  the  radius  of  the  earth  to  the  height  of  the  atmosphere, 
because,  while  the  greatest  length  of  the  horizontal  current  is  from  the 
equator  to  the  pole,  that  of  the  vertical  current  is  from  the  earth's 
surface  to  the  upper  limit  of  the  atmosphere. 

The  Effect  of  the  Earth's  Rotation,  on  the  air  currents 
passing  between  the  equator  and  the  poles  along  meridians, 
and  on  the  vertical  air  currents  which  connect  these  hori- 


196 


ELEMENTARY    METEOROLOGY. 


zontal  currents,  is  a  matter  which  has  not  yet  been  inves- 
tigated in  all  of  its  details,  and  it  is  so  difficult  that  only  a 
meager  outline  of  it  can  be  presented  here.  There  is  one 
general  theorem  on  which  are  based  all  modern  explana- 
tions of  this  action,  and  this  is  as  follows  :  — 

If  a  free-moving  particle  (such  as  air)  moves  along  near 
the  earth's  surface,  there  is  a  force  arising  from  the  diurnal 
rotation  of  the  earth  which  deflects  it  to  the  right  of  its  course 
in  tJie  northern  hemisphere,  and  to  the  left  of  its  coiirse  in 
the  southern  hemisphere.  The  amount  of  this  force  in- 
creases with  the  mass  and  the  velocity  of  the  particle,  and 

also  with  the  increase  of 
latitude.  It  is  zero  at  the 
equator,  and  greatest  at 
the  poles.1 

The  following  illustration 
will  show  the  general  nature 
of  this  deflecting  force  :  — 

In  Fig.  60  we  have  a  view 
of   the    northern    hemisphere 
as  it  is  seen  from  above  the 
north  pole.     The  direction  of 
axial    rotation   is   that   shown 
FIG.  60.  —  DEFLECTION  OF  THE  WINDS  BY  EARTH'S     by  the  curved  arrows  along  the 
ROTATION.  outer   circle   representing    the 

equator.  This  direction  is  opposite  to  that  of  the  hands  of  a  watch. 
Suppose  that  at  A  there  is  a  wind  blowing  along  the  meridian  towards 
the  north  pole,  as  shown  by  the  arrow  a.  While  the  meridian  at  A  is 
advancing  to  the  position  B  (due  to  the  diurnal  rotation  of  the  earth), 
the  air  which  was  at  A  has  been  moving  in  a  straight  line,  that  is, 
in  the  direction  indicated  by  the  arrow  a,  so  that  this  same  direction 

1  The  formula  expressing  the  amount  of  this  force  is  2  MVW 'sin  D,  where 
J/is  the  mass,  Fthe  velocity,  W  the  angular  rotation  of  the  earth  on  its  axis, 
and  D  the  latitude.  The  meaning  of  this  formula  is  too  complex  for  explana- 
tion in  this  elementary  treatise,  but  is  fully  given  in  Ferrel's  "  Winds." 


GENERAL   CIRCULATION   OF  THE   ATMOSPHERE.  197 

when  the  meridian  reaches  the  position  B  will  be  indicated  by  the  arrow 
b  parallel  to  the  arrow  a.  But  it  is  seen  that  this  direction  of  the  arrow 
b  is  to  the  right  of  the  meridian  A  in  this  new  position  B.  Similarly, 
when  the  meridian  A  has  assumed  the  positions  C  and  D,  the  air  will 
have  a  motion  in  the  directions  shown  by  the  arrows  c  and  d. 

Another  set  of  arrows  on  the  right  hand  shows  the  effect  of  the 
deflection  (due  to  the  earth's  rotation)  on  air  motions  towards  the  west ; 
another  set  of  arrows  at  the  top  shows  the  effect  on  motions  from  the 
pole  towards  the  equator ;  and  still  another  set  of  arrows  on  the  left 
shows  the  effect  on  motions  towards  the  east. 

A  revolving  terrestrial  globe  with  a  fixed  brass  meridional  scale  gives 
a  still  clearer  idea  of  this  matter  if  one  takes  a  pencil  in  hand  as  if 
for  writing,  rests  the  hand,  by  means  of  the  end  of  the  little  finger, 
on  the  globe,  and  then  slowly  revolves  the  globe  from  west  to  east, 
letting  the  hand  move  with  it,  but  at  the  same  time  giving  a  slight  move- 
ment to  the  pencil  point  (as  in  writing),  and  keeps  the  latter  moving  in 
a  direction  either  perpendicular  or  parallel  to  the  fixed  brass  meridian 
scale.  The  path  traced  out  by  the  pencil  point  will  show  the  direction 
of  deviation  which  the  moving  air  would  take  for  an  east-westerly  or 
north-southerly  initial  direction.  Any  other  directions  than  the  four 
cardinal  ones  mentioned  can  be  followed  out  by  making  the  path  of  the 
pencil  always  keep  a  fixed  angle  with  the  brass  meridian  scale. 

The  amount  of  the  influence  of  the  rotation  of  the  earth  is  best 
shown  by  an  example.  At  latitude  50°,  a  rifle  ball  moving  1,700  feet 
per  second,  discharged  at  a  target  3,300  feet  distant,  would  deviate  about 
4  inches  to  the  right  of  the  target  in  the  northern  hemisphere,  and 
the  same  amount  to  the  left  of  the  target  in  the  southern  hemisphere. 
This  effect  may  seem  slight ;  but  when  the  force  operates  on  masses 
of  air  moving  day  after  day,  and  over  distances  of  thousands  of  miles, 
its  results  are  of  great  importance. 

Effect  of  the  Deflecting  Force  on  the  Polar  and  Equatorial 
Air  Currents.  —  The  qualitative  effect  of  this  force  on  the 
horizontal  meridional  motions  which  have  been  described, 
is  to  cause  the  free-moving  air  currents  to  depart  to  the 
right  of  their  course  in  the  northern  hemisphere,  and  to 
the  left  in  the  southern  hemisphere. 

The  upper  poleward  currents  are,  then,  deflected  towards 

WALDO  METEOR.  —  12 


198  ELEMENTARY   METEOROLOGY 

the  east,  and  tne  lower  currents  flowing  towards  the  equa- 
tor are  deflected  towards  the  west,  in  both  hemispheres. 
We  thus  see  that  there  will  be  opposite  directions  of  mo- 
tions high  in  the  air  and  near  the  earth's  surface,  not  only 
in  a  north-and-south  direction,  but  also  in  an  east-and- 
west  direction  (the  upper  horizontal  current  tending 
towards  the  northeast,  and  the  lower  one  towards  the 
southwest,  in  the  northern  hemisphere) ;  and  the  friction, 
and  the  intermingling  of  the  air  which  arises  at  the 
boundary  of  these  oppositely  directed  motions,  are  very 
important  in  their  effects.  There  results  then  a  gyratory 
motion  around  the  pole,  toward  it  aloft,  and  away  from  it 
below ;  and,  from  the  law  of  conservation  of  areas,  the  ve- 
locities aloft  tend  to  increase,  and  those  below  to  decrease. 

Easterly  Motion  of  the  Air  in  High  (North)  Latitudes.  - 
Since  the  effects  of  this  deflecting  force  accumulate  as  long 
as  the  current  continues,  therefore  in  northern  latitudes  the 
motion  to  the  right,  or  easterly  motion,  of  the  upper  poleward 
current,  becomes  very  great  in  comparison  with  the  westerly 
motion  at  the  beginning  of  the  equatorial  current  beneath 
it.  So  that  this  upper  easterly  motion,  by  friction,  and 
especially  by  intermingling  of  the  air,  gradually  entirely 
overpowers  the  lower  westerly  motion,  and  a  general  easterly 
motion  results  in  both  the  upper  and  lower  layers  of  the 
atmosphere  in  these  latitudes.  This  general  eastward  air 
movement,  it  is  found,  actually  takes  place  around  the  pole. 

Westerly  Motion  of  Air  in  Low  (North)  Latitudes.  —  In 
passing  towards  the  equator,  however,  the  lower  equatorial 
current  gains  strength  in  its  endeavors  to  flow  towards  the 
west,  in  obedience  to  the  force  carrying  it  towards  the 
right;  and  after  reaching  a  certain  latitude  it  is  able, 
through  friction  and  the  intermingling  of  air  in  the  cur- 
rents, to  just  neutralize  the  (to  that  point  overpowering) 


GENERAL   CIRCULATION   OF  THE  ATMOSPHERE.  199 

effects  of  the  upper  eastward  current,  so  that  there  will  be 
no  east  or  west  motion  in  the  lower  air  layers.  Passing 
still  farther  south,  however,  the  effects  of  the  force  towards 
the  west  increase,  and  a  westerly  air  current  arises  at  low 
altitudes ;  and  as  lower  latitudes  are  reached,  the  power  of 
this  western  current  becomes  relatively  so  great  as  to  over- 
come the  upper  easterly  current,  which  is  weak  there ;  and 
in  very  low  latitudes  a  general  westerly  motion  of  the  air 
takes  place  at  all  altitudes.  So  that  very  near  the  equator 
there  is  a  wind  directed  a  little  to  the  north  of  west  in  the 
upper,  and  a  little  to  the  south  of  west  in  the  lower,  air 
layers. 

The  Limits  of  Velocities  of  Air  Currents.  -^  Since  the  deflecting 
effect  of  the  earth's  rotation  is  zero  at  the  equator  and  increases 
with  the  latitude,  then  the  poleward  upper  current  is  more  and  more 
deflected  towards  the  east  as  it  moves  northward ;  and  since  the  cause 
of  the  original  motion  acts  along  the  meridians  from  the  equator 
towards  the  north,  then  the  deflection  towards  the  right,  due  to  the 
earth's  rotation,  must  be  directed  partly  against  this  movement,  and 
must  have  a  retarding  influence  on  it.  This  deflecting  force  increases 
with  the  increase  in  velocity ;  and  it  is  evident  that  if  the  velocities  in- 
creased enough,  there  would  be  a  deflecting  force  towards  the  right  (in 
this  case  towards  the  equator)  sufficient  to  counterbalance  the  force 
producing  the  original  poleward  current,  and  this  motion  would  cease. 
We  thus  see  that  there  is  a  practical  limit  to  the  velocities  which  the 
polar  current  may  attain ;  for,  if  these  increase  too  much,  the  deflection 
to  the  right  will  also  increase  to  such  an  extent  as  to  actually  over- 
come the  poleward  current  in  its  course.  This  acts  like  a  controlling 
governor  on  the  air  circulation. 

A  second  control  over  excessive  air  motions  is  the  fact  that  when  a 
current  is  impelled  by  a  constantly  acting  force,  the  current  is  contin- 
ually breaking  up  into  relatively  small  atmospheric  whirls  (analogous 
to  the  whirls  in  flowing  water)  which  impede  the  regular  progress  of 
the  current  by  mixing  up  the  various  air  masses. 

A  third  impediment  to  excessive  velocities  is  the  frequent  occurrence 
of  vertical  currents  in  the  air. 


2OO  ELEMENTARY   METEOROLOGY. 

Vertical  Air  Movements  in  the  General  Circulation.  — The 

vertical  currents  in  the  general  atmospheric  circulation  — 
those  which  connect  the  horizontal  currents  —  are  the 
descending  current  in  polar  latitudes,  and  the  ascending 
current  in  the  neighborhood  of  the  equator.  Two  addi- 
tional vertical  currents  shown  in  Fig.  55  at  about  latitude 
30°  are  explained  on  p.  203  in  connection  with  the  effects 
of  the  air-pressure  distribution  on  the  general  circulation 
of  the  air. 

Downward  Current  at  High  Latitudes.— The  upper  air,  as 
it  approaches  the  pole,  begins  to  descend,  or  settle  down 
towards  the  earth,  and  the  comparatively  great  easterly 
motion  which  the  air  has  in  this  region  is  mostly  used  up 
in  overcoming  the  friction  between  the  successive  air 
layers  as  the  air  makes  its  way  downward ;  but  aloft  there 
is  a  sufficiently  powerful  motion  towards  the  east  to  give 
an  easterly  tendency  to  the  air  even  at  the  surface  of  the 
earth  in  this  region.  Moreover,  when  the  air  descends 
in  the  higher  latitudes,  it  approaches  a  little  nearer  to 
the  axis  of  rotation  of  the  earth ;  and  its  velocity  becomes 
slightly  increased,  according  to  the  conservation  of  areas. 

Upward  Current  at  Low  Latitudes.  —  The  upward  cur- 
rent, which  extends  to  a  considerable  distance  from  the 
equator  poleward,  must  in  its  lower  course  have  a  motion 
towards  the  west  imparted  to  it  by  the  lower  horizontal  air 
current  flowing  towards  the  equator.  This  west  component 
of  motion  decreases  with  the  altitude  up  to  a  certain  height 
above  the  earth's  surface,  where  it  changes  into  an  east- 
ward motion  ;  for  at  high  altitudes  there  is  an  easterly  cur- 
rent over,  the  whole  region  from  the  equator  to  the  pole, 
except,  perhaps,  directly  over  the  equator,  where  the 
westerly  motion  extends  up  to  very  high  altitudes.  The 
altitude  where  the  change  of  direction  takes  place  de- 


GENERAL   CIRCULATION   OF  THE   ATMOSPHERE.  2OI 

creases  up  to  about  latitude  30°,  beyond  which  the  wind 
becomes  generally  easterly  (towards  the  east). 

Effect  of  Air  Motions  on  Barometric  Pressure.  —  The 
effect  of  these  general  motions  of  the  atmosphere  on  the 
distribution  of  the  air  pressure  and  the  isobaric  surfaces 
is  of  great  importance.  If  the  air  temperature  were 
everywhere  the  same,  then  the  isobaric  surfaces  would 
be  level.  Now,  we  have  seen  that  by  heating  the  air 
at  the  equator  the  successive  isobaric  surfaces  are  ele- 
vated there ;  but  directly  at  the  surface  of  the  earth 
there  is  no  elevation  of  the  isobaric  surface,  and  there- 
fore there  is  no  pressure  gradient  at  the  earth's  surface 
such  as  exists  at  various  altitudes  above  it.  But  when 
there  are  easterly  or  westerly  air  currents  at  (or  close  to) 
the  earth's  surface,  this  uniformity  of  the  isobaric  sur- 
face at  the  earth's  surface  no  longer  exists,  but  there  arise 
surface  gradients ;  and  where  there  are  no  east  or  west 
motions,  there  are  no  gradients  at  the  earth's  surface. 

The  Barometric  Gradients  at  the  Earth's  Surface  are 
accounted  for  by  the  action  of  the  centrifugal  force  which 
arises  from  the  eastward  (or  the  westward)  movement  of 
the  air,  this  movement  being  equivalent  to  a  whirling  mo- 
tion around  the  pole  as  a  center  (or,  in  more  local  whirls, 
around  some  other  point  as  a  center).  Whenever  this 
whirling  motion  occurs,  the  particles  so  moved  tend  to 
leave  their  curvilinear  track,  owing  to  the  centrifugal  force. 

In  the  case  of  a  whirling  mass  of  water  such  as  is 
to  be  seen  in  any  whirlpool,  the  water  particles  are 
free  to  move,  and  there  is  a  heaping-up  of  the  water 
around  but  away  from  the  center  of  the  whirl,  and  a  de- 
crease of  the  water  towards  the  center  of  the  whirl.  The 
water  recedes  from  the  center  to  a  distance  such  that  the 
centrifugal  force  causing  it  to  move  away  is  just  equal  to 


202  ELEMENTARY   METEOROLOGY. 

the  downward  push,  or  gradient  force,  of  the  heaped-up 
water  towards  the  center  of  the  whirl. 

In  the  case  of  the  air  motion  around  the  north  pole, 
which  is  the  center  of  the  whirl,  the  current  towards 
the  east,  in  the  middle  and  northern  latitudes,  gives  rise  to 
the  action  of  centrifugal  force,  which  causes  a  heaping-up 
of  the  air  at  the  latitudes  away  from  the  pole,  and 
consequently  to  the  right  and  to  the  equatorward  of  the 
easterly  current,  and  a  depletion  or  lessening  of  the  air  on 
the  inner,  the  left  or  polar,  side  of  the  current.  Thus  we 
have  a  lessening  pressure  towards  the  pole,  just  as  there 
is  towards  the  center  of  a  water  whirlpool. 

The  air  on  the  equatorial  side  is  thus  thrown  off  by 
the  centrifugal  force,  and  distributes  itself  over  the 
equatorial  regions  of  the  northern  hemisphere,  where  it 
meets  with  a  corresponding  heaping-up  of  the  air  coming 
from  the  polar  whirl  of  the  southern  hemisphere. 

Barometric  Pressure  Gradients  at  Various  Altitudes. — 
Up  at  considerable  altitudes  above  the  earth's  surface,  this 
heaping-up  of  the  air  and  increase  of  pressure  take  place 
clear  to  the  equator,  as  just  mentioned ;  so  that  at  high 
altitudes  there  is  a  maximum  pressure  at  the  equator,  and 
a  continuous  decrease  from  thence  to  the  pole.  But  the 
maximum  pressure  at  the  earth's  surface  is  found  at  about 
latitude  30°,  where  the  easterly  motion  ceases,  and  there  is 
no  east  or  west  motion.  When,  however,  the  westward 
motion  sets  in,  as  we  pass  from  this  point  towards  the 
equator,  then  there  is  a  slight  decrease  in  the  pressure, 
due  to  the  opposing  centrifugal  effects  of  the  westerly  wind. 
It  is  not  so  much  a  real  decrease  in  the  pressure  as  it  is  a 
lessening  of  the  pressure  which  would  exist  if  the  easterly 
wind  alone  existed  all  the  way  to  the  equator.  This  is 
brought  out  more  plainly  in  the  variation  of  the  pressure 


GENERAL   CIRCULATION   OF  THE   ATMOSPHERE.  203 

with  the  latitude  in  the  southern  hemisphere  (see  t  table, 
p.  96),  where  the  disturbing  influence  of  the  land  is  not  so 
markedly  felt. 

The  maximum  pressure,  then,  is  to  be  found  at  about 
latitude  30°  at  the  earth's  surface,  but  with  increasing 
altitude  it  lies  nearer  and  nearer  to  the  equator.  Up 
above  this  dividing  line  of  maximum  pressure  there  is  a 
motion  from  the  west,  probably  all  the  way  to  the  equator, 
and  over  the  lower  westerly  motion  which  exists  between 
the  equator  and  about  latitude  30°.  This  allows,  then,  a 
motion  towards  the  east  up  at  high  altitudes  in  all  latitudes. 

Cause  of  the  Equatorial  and  Poleward  Surface  Winds  in 
Lower  Middle  Latitudes.  —  In  the  meridional  projection  of 
the  general  air  circulation  (Fig.  55)  the  poleward  motion 
is  represented  above,  with  the  equatorward  motion  along 
the  surface  below  it ;  but  there  is  still  a  peculiarity 
which  must  be  mentioned,  and  which  can  be  best  illus- 
trated by  the  diagram.  It  is  seen  that  (on  the  meridi- 
onal section)  at  about  latitude  30°  there  is  a  current 
along  and  near  the  surface  of  the  earth  moving  towards 
the  pole,  which  is  contrary  to  what  might  be  expected ; 
and  another  towards  the  equator,  although .  this  is  in  the 
direction  to  be  expected.  These  currents  have  as  their 
cause  the  heaping-up  of  the  air  shown  by  the  ring  of 
high  pressure  at  latitude  30°.  The  atmosphere  here  exerts 
a  greater  downward  pressure  than  that  either  side  of  it, 
and  so  forces  out  the  air  from  the  lower  layers  beneath,  in 
the  attempt  at  equalizing  the  pressure.  Part  of  the  air 
from  this  lower  layer  is  then  forced  poleward,  and  part 
equatorward.  These  currents  are  below  the  great  equa- 
torial return  current,  and  are  fed  by  a  downward  vertical 
current  in  the  lower  air  layers :  they  are  indicated  in  Fig. 
55  at  the  latitudes  marked  "  Tropical  Calm  and  Dry  Belt.'* 


2O4 


ELEMENTARY   METEOROLOGY. 


The  Air  Circulation,  viewed  as  a  Whole,  can  be  considered 
as  consisting  of  two  huge  atmospheric  whirls  with  the 
poles  as  centers  (Fig.  61).  The  direction  of  rotation  of 
these  whirls  is  determined  by  the  rotation  of  the  earth  on 
its  axis ;  in  the  northern  hemisphere  it  is  opposite  to  the 
movement  of  the  hands  of  a  watch  (face  upwards),  and 
in  the  southern  hemisphere  the  direction  is  the  same  as 
that  of  the  hands  of  a  watch.  At  the  outer  edge  of 
each  of  these  whirls  (that  is,  between  the  tropical  region 
and  the  equator)  there  is  an  atmospheric  ring  or  belt, 


•Equator  Equator 

Lower  Currents  Upper  Currents 

FIG.  61.— GENERAL  HEMISPHERICAL  DIRECTION  OF  HORIZONTAL  AIR  CURRENTS. 

decreasing  in  latitudinal  width  with  increase  of  altitude 
above  the  earth's  surface,  in  which  the  motion  of  rotation 
is  in  the  opposite  direction  to  that  of  the  respective  whirls. 
There  is  an  outward  flow  of  air,  at  middle  altitudes,  from 
the  polar  centers  of  these  whirls.  In  the  transition  zone 
between  this  inner  (polar)  region  and  outer  (equatorial) 
ring  with  opposite  directions  of  rotation,  there  is  a  heap- 
ing-up  of  the  air,  and  a  consequent  increase  of  the  air 
pressure,  due  to  the  centrifugal  force  of  these  motions. 
The  moving  air  obeys  the  law  of  the  conservation  of 
areas  (see  p.  182). 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE.  2O5 

An  Interchange  of  Air  between  the  Northern  and  Southern 
Hemispheres  also  takes  place,  owing  to  the  transference  of 
the  region  of  greatest  temperature,  first  to  one  side,  and 
then  to  the  other  side,  of  the  equator,  with  the  annual 
movement  of  the  sun.  When,  for  instance,  we  have  our 
winter  in  the  northern  hemisphere,  and  summer  exists  in 
the  southern  hemisphere,  then  the  isobaric  surfaces  are 
more  elevated  by  the  intenser  heat  and  subsequent  expan- 
sion of  the  air  over  the  equatorial  regions  in  the  southern 
hemisphere  than  in  the  northern  hemisphere,  and  air  flows 
down  these  surfaces  from  the  southern  into  the  northern 
hemisphere.  Similarly,  when  we  have  our  summer  in  the 
northern  hemisphere,  there  is  a  flow  of  air  across  the  equa- 
tor from  the  northern  into  the  southern  hemisphere. 

Inequality  in  Air  Motions  Due  to  Variations  in  Tempera- 
ture. —  The  difference  in  temperature  between  the  polar 
and  equatorial  regions  is  much  greater  in  the  winter  time 
than  in  the  summer  time,  and  consequently  the  slopes  of 
the  isobaric  surfaces  are  greater ;  and  since  the  velocities 
of  the  air  currents  depend  on  the  gradient  or  slope  of 
these  surfaces,  these  velocities  are  much  greater  in  win- 
ter than  in  summer.  From  the  observed  data  of  wind 
velocities  it  is  seen  that  the  winter  velocities  are  in  many 
cases  double  those  of  the  summer. 

Theoretical  Computation  of  Easterly  and  Westerly  Velocities  of  the 
Wind. — Attempts  have  been  made  to  compute  theoretically  the  east- 
erly and  westerly  velocities  of  the  wind  at  various  altitudes  on  the 
different  parallels  of  latitude,  by  means  of  the  gradient  forces  result- 
ing from  the  inclination  of  the  isobaric  surfaces  due  to  differences  in 
temperature  and  air  pressure  between  the  equator  and  the  poles  along 
an  average  meridian.  These  velocities  in  miles  per  hour,  for  the  yearly 
average,  are  given  on  p.  206. 

It  is  seen  from  this  table  that  the  westerly  motions  disappear  at  a 
height  of  about  10,000  feet  above  sea  level  to  the  poleward  of  about 


206 


ELEMENTARY   METEOROLOGY. 


COMPUTED  EASTERLY  OR  WESTERLY  WIND  VELOCITIES  ALONG  A 
MERIDIAN. 


LATITUDE. 

EASTERLY  (E.)  OR  WESTERLY  (W.) 
VELOCITY  OF  WIND  IN  MILES  PER 
HOUR,  AT  VARIOUS  ALTITUDES. 

INCREASE  IN  EASTERLY  VELOCI- 
TIES WITH  EACH  (ABOUT)  3,300 
FEET  IN  ALTITUDE. 

Sea  level. 

About 
3,300  feet. 

About 
13,200  feet. 

Miles  per  hour. 

N.  Lat. 

75° 

W.     2.7 

E.        0.2 

E.      9.2 

E.  +  3-° 

70° 

W.      2.0 

E.        2.0 

E.     14.3 

E.      4.1 

65° 

E.     o.i 

E.      4.9 

E.    19.3 

E.     4.8 

60° 

E.      2.4 

E.      7.6 

E.    23.1 

E.     5.2 

55° 

E.      3.4 

E.      8.7 

E.    24.5 

E.     5-3 

50° 

E.      3-3 

E.      8.7 

E.    24.9 

E.      54 

45° 

E.      3.0 

E.      8.5 

E.    25.0 

E.     5-5 

40° 

E.      1.6 

E.      7.2 

E.    24.0 

E.     5.6 

35° 

W.     0.7 

E.      5.0 

E.    22.4 

E.      5.8 

30° 

w.    5-3 

E.      0.6 

E.    18.2 

E.     5-9 

25° 

W.     8.9 

W.     3.1 

E.    14.4 

E.      5.8 

20° 

W.     9-4 

W.     3.8 

E.    13.0 

E.      5.6 

N.  Lat. 

15° 

W.     7.8 

W.     4.3 

E.      6.1 

E.      3-5 

Equator 

0° 

S.  Lat. 

15° 

W.   15.6 

W.   10.5 

E.      4.8 

E.      5.1 

20° 

W.   13.0 

W.     8.2 

E.      6.4 

E.      4.8 

25° 

W.     6.4 

W.      1.7 

E.    12.5 

E.     4-7 

30° 

E.      2.4 

E.      7.0 

E.     21.0 

E.      4.7 

35° 

E.      7.7 

E.    12.3 

E.    26.1 

E.     4.6 

40° 

E.    1  1.  6 

E.     1  6.  2 

E.    30.0 

E.      4.6 

45° 

E.    14.9 

E.    19.5 

E.    33-3 

E.     4-6 

50° 

E.    17.1 

E.    21.7 

E.    35-7 

E.      4.6 

55° 

E.    17.0 

E.    21.6 

E.    35-6 

E.     4-7 

S.  Lat. 

60° 

E.    13.6 

E.    18.2 

E.    32.2 

E.+4-7 

latitude  15°.  And  we  see  here,  derived  by  theory,  the  observed  fact 
of  the  easterly  wind  velocity  near  the  earth's  surface  in  the  northern 
hemisphere  reaching  a  maximum  at  about  latitude  50°  or  60°  ;  but  it  is 
to  be  noticed  that  this  region  of  greatest  velocity  decreases  in  latitude 
with  increase  of  altitude. 


GENERAL   CIRCULATION   OF  THE   ATMOSPHERE.  207 

Observed  Circulation  of  the  Atmosphere. — Usually  the 
arrows  showing  the  direction  of  the  winds  on  charts  are 
drawn  partly  from  direct  observation,  and  partly  from  the 
known  laws  of  the  direction  of  wind  as  depending  on  the 
distribution  of  the  air  pressure.  Such  are  the  indications 
of  wind  directions  given  on  the  charts  of  air  pressure  for 
the  globe  (see  Figs.  26,  27). 

The  lower  air  currents  are  difficult  to  observe  on  the  continents, 
owing  to  the  irregularities  of  the  ground,  and  so  we  must  turn  to  the 
observations  made  on  the  ocean  for  the  best  obtainable  presentation  of 
their  characteristics.  Published  charts  showing  the  general  wind  direc- 
tion for  the  Atlantic  Ocean  have  been  already  given  (see  Figs.  36,  37). 
It  is  seen  on  these,  that  while  the  surface  wind  directions  follow  in 
general  those  deduced  by  theoretical  reasoning  and  shown  in  the  pre- 
vious sections,  yet  there  are  localities  in  which  the  local  pressure  con- 
ditions exert  such  a  predominating  influence  that  the  main  currents  are 
interrupted.  Thus,  on  the  northern  North  Atlantic  and  middle  South 
Atlantic  oceans,  there  are  local  atmospheric  whirls  formed  in  the  win- 
ter of  the  northern  hemisphere  ;  and  in  the  summer  there  is  still  another 
interrupting  whirl  formed  on  the  middle  North  Atlantic. 

In  the  northern  hemisphere,  the  observed  general  direc- 
tion of  the  upper  air  currents  is  from  N.  45°  W.  to 
S.  45°  E.,  in  high  latitudes  like  Lapland.  In  middle  lati- 
tudes (at  about  latitude  40°)  the  direction  is  nearly  due 
easterly.  In  lower  latitudes  (at  about  latitude  25°)  it  is 
perhaps  from  S.  75°  W.  to  N.  75°  E.  In  the  equatorial 
regions,  however,  the  direction  is  in  general  from  the 
east  towards  the  west,  and  in  some  cases  it  is  from 
S.  45°  E.  to  N.  45°  W. ;  but  there  is  a  much  greater 
irregularity  of  direction  in  this  region  than  near  the  poles, 
on  account  of  the  shifting  of  the  region  of  greatest  heat 
from  one  hemisphere  to  another,  There  exist  compara- 
tively few  wind  observations,  on  the  land,  for  the  inter- 
tropical  regions. 


208 


ELEMENTARY   METEOROLOGY. 


Trade  Winds.  —  These  are  the  lower  winds  belonging  to 
that  part  of  the  general  atmospheric  circulation  embraced 
between  the  equatorial  region,  of  calms  and  the  tropical 
belt  of  high  atmospheric  pressure.  They  blow  from  the 
northeast  in  the  northern  hemisphere,  and  from  the  south- 
east in  the  southern  hemisphere.  The  general  location 
and  direction  of  these  winds  are  shown  on  the  diagram  of 
the  general  circulation  of  the  atmosphere  (see  Fig.  55), 
where  they  are  indicated  by  heavy  arrows  between  the 
Tropical  and  Equatorial  Calm  Belts.  They  have  not  very 
great  velocities,  but  they  are  relatively  very  constant  in 
their  continuance  and  in  their  velocity  of  motion.  The 
whole  system  of  trade  winds  shifts  from  north  to  south, 
and  the  reverse,  following  the  sun  in  its  course ;  and  it 
also  varies  somewhat  in  width,  not  only  with  the  longi- 
tude, but  also  with  the  season  of  the  year,  the  latter  being 
shown  by  the  following  table :  — 

LATITUDINAL  LIMITS  OF  THE  TRADE  WINDS. 


AVERAGE  OVER  THE  PACIFIC  AND  ATLANTIC  OCEANS. 

IN    MARCH. 

IN   SEPTEMBER. 

In   northern    hemisphere  "t 
(northeast  trades)           / 
In  southern    hemisphere  1 
(southeast  trades)           / 

Lat.  4°  N.  to  25    X. 
Lat.  1°  N.  to  26°  S. 

Lat.  10°  N.  to  33°  N. 
Lat.    5°  N.  to  22    S. 

Doldrums.  —  Between  the  surface  winds  of  the  two 
hemispheres  there  is  an  equatorial  region  of  calms,  called 
the  doldrums.  This  region  lies  a  little  to  the  north  of  the 
terrestrial  equator ;  but  it  varies  somewhat  in  locality  and 
width,  not  only  regularly  with  the  season  of  the  year, 
but  also  irregularly  with  the  longitude.  In  March  the  dol- 


GENERAL   CIRCULATION   OF  THE  ATMOSPHERE.  2OQ 

drums  extend  from  o°  to  3°  north  latitude  on  the  Atlantic 
Ocean,  and  from  3°  to  5°  north  latitude  on  the  Pacific 
Ocean;  in  September  they  extend  from  3°  to  n°  north 
latitude  on  the  Atlantic  Ocean,  and  from  7°  to  10°  north 
latitude  on  the  Pacific  Ocean. 

Monsoons,  or  continental  land  and  sea  winds,  occur  dur- 
ing the  hot  and  cold  seasons  of  the  year.  During  the  sum- 
mer the  interiors  of  the  continents  become  heated,  and  this 
elevates  locally  the  surfaces  of  equal  air  pressure,  and  the 
air  flows  down  these  towards  the  ocean  at  some  distance 
above  the  ground;  and  a  countercurrent  near  the  ground 
sets  in  from  the  ocean  towards  the  land.  In  the  winter 
time  the  opposite  interchange  of  air  takes  place,  and  along 
the  ground  the  air  flows  from  the  land  towards  the  sea. 

In  case  of  an  uninterrupted  low-level  region  extending  from  the 
ocean  inland,  the  monsoon  effects  are  not  so  marked  as  when  the  inte- 
rior is  high  or  mountainous.  One  reason  for  this  is,  that  where  there 
is  a  long  surface  slope  the  conditions  are  best  for  an  extended  general 
movement  of  the  air.  In  winter  the  lower  air  will  flow  downward  much 
easier  than  on  a  level ;  and  in  summer  the  warm  air  flows  upward  along 
the  slopes  more  easily  than  along  a  level  surface,  just  as  it  will  rise 
better  in  a  somewhat  vertical  flue  than  move  in  a  horizontal  flue ;  and 
there  is  less  tendency  for  local  currents  to  form.  Another  reason  is 
the  fact  that  the  temperatures  on  elevated  land,  especially  on  plateaus, 
are  greater  in  summer  and  less  in  winter  than  at  the  same  altitudes 
above  the  ocean  or  the  low  land ;  and  there  is  .thus  a  contrast  in  the 
temperatures  maintained  to  a  height  depending  on  the  altitudes 
attained  by  the  land.  This  principle  is  illustrated  by  the  greater 
air  circulation  (or  draught)  in  the  case  of  a  tall  chimney  as  compared 
with  a  short  one.  This  effect  is  felt  mostly  in  the  summer  monsoon, 
as  in  winter  the  air  temperature  of  the  land  is  not  so  much  below  that 
of  the  surrounding  air  as  it  is  above  in  summer. 

While  the  monsoon  winds  occur  in  many  regions,  yet  nowhere  else 
are  the  conditions  so  favorable  as  in  India  and  the  north  Indian  Ocean. 
There  the  low  lands  are  backed  by  the  Himalaya  mountain  range,  to 
the  north  of  which  Ires  the  Kuenlun  range,  with  the  plateau  of  Tibet 


(210) 


(211) 


212  ELEMENTARY  METEOROLOGY. 

between ;  and  still  farther  to  the  north  lie  vast  stretches  of  quite  high 
plateau  desert  land.  During  the  summer  monsoon  the  region  of  great 
surface  heat  of  the  Indian  Ocean  and  the  adjacent  low  lands,  which 
consequently  affords  the  air  an  excessive  capacity  for  moisture,  lies 
close  to  the  elevated  region  just  described ;  and  when  the  air  moves 
u.p  the  steep  slopes,  as  a  result  of  the  adiabatic  cooling  excessive  pre- 
cipitation takes  place,  and  enormous  quantities  of  freed  latent  heat 
increase  the  temperature  of  the  air  over  the  continent  above  what  it 
would  be  in  the  case  of  no  precipitation,  and  the  wind  velocity  is 
thereby  increased. 

Strongly  developed  monsoon  winds  so  act  that  they 
greatly  influence,  and  may  even  reverse,  the  direction 
of  the  regular  primary  wind  circulation. 

The  accompanying  figures  (Figs.  62,  63)  show  the  very  powerful 
monsoon  winds  of  the  north  Indian  Ocean  and  southern  Asia  in 
midwinter  and  midsummer.  Heavy  arrows  indicate  strong  winds ; 
light  arrows,  light  winds ;  and  circles,  calms.  Regions  of  variable 
winds  are  denoted  by  short  arrows,  while  the  long  arrows  designate 
steady  winds.  In  January  and  February  the  outflow  of  air  from  the 
interior  of  the  Asiatic  continent  is  southward  across  the  north  Indian 
Ocean  4:o  10°  of  latitude  beyond  the  equator.  In  July  and  August  there 
is  an  inflow  of  air  towards  the  interior  of  the  Asiatic  continent,  not  only 
extending  to  the  equatorial  region  of  the  Indian  Ocean,  but  also  form- 
ing a  continuation  of  the  winds  of  the  lower  latitudes  of  the  southern 
hemisphere  (to  30°  south  latitude)  which  flow  in  towards  the  equator. 
This  continuous  northward  sweep  of  the  winds  over  the  ocean  from  30° 
south  latitude  to  over  20°  north  latitude,  caused  by  the  great  indraught 
over  the  heated  continent,  readily  accounts  for  the  excessive  force  of 
the  southern  monsoon  winds  of  midsummer  on  the  Indian  seas  and 
mainland. 


CHAPTER    IX. 
SECONDARY  CIRCULATION  OF  THE   ATMOSPHERE. 

WE  must  next  examine  those  more  local  conditions  of 
the  atmospheric  circulation  which  interrupt  the  general 
circulation,  and  occasion  the  variability  in  the  winds  during 
brief  periods  of  time. 

We  will  take  up  those  conditions  which  are  most  closely 
connected  with,  and  to  a  great  extent  depend  on,  the  cur- 
rents of  the  general  air  circulation.  Such  are  the  local 
atmospheric  whirls  which  occur  irregularly  in  the  greater 
hemispherical  currents,  and  which  appear  to  gradually 
form  in  the  greater  current,  follow  it  in  its  general  course 
for  a  time,  develop  in  intensity  to  maturity,  and  then 
gradually  disappear. 

Movement  of  Air  in  Whirls.  —  It  is  a  marked  charac- 
teristic of  atmospheric  motion  that  it  occurs  mainly  in  the 
form  of  whirls  or  vortex  motion.  Even  when  the  wind 
blows  in  an  apparently  straight  line,  if  it  is  followed  out 
far  enough,  it  will  usually  be  found  to  belong  to  some 
system  of  whirling  motions.  Thus  the  great  air  currents 
forming  the  general  circulation  of  the  atmosphere  have 
been  found  to  be  but  a  part  of  the  great  hemispherical 
vortical  movement  described  in  the  preceding  chapter. 

Secondary  Atmospheric  Whirls. — Within  these  mighty 
air  currents  of  the  hemispherical  air  circulation  there  exist 
limited  systems  of  atmospheric  whirls  in  which,  in  some 
cases,  the  air  moves  spirally  inward  towards  a  center,  and 

WALDO    METEOR. —  13  21^ 


214 


ELEMENTARY    METEOROLOGY. 


in  others  spirally  outward  from  a  center.  In  such  local 
whirls  there  exist  gradients  of  air  pressure  such  as  occur 
in  the  great  hemispherical  whirl :  and  these  may  be 
arranged  in  two  ways,  —  first,  there  may  be  an  area  of 
lower  atmospheric  pressure  at  the  center  of  the  whirl,  in 
which  case  the  air  movement  is  spirally  inward,  and  such 
a  system  is  called  a  cyclone  ;  second,  there  may  be  a  region 
of  higher  atmospheric  pressure  at  the  center,  in  which 
case  the  spiral  air  movement  is  outward  from  the  central 
area,  and  such  a  system  is  called  an  anticyclone  (Fig.  64). 


FIG.  64.  —  DIRECTION  OF  WHIRL  FOR  A  CYCLONE  AND  AN  ANTICYCLONE  (NORTHERN 
HEMISPHERE). 

Near  the  center  of  a  cyclone  there  is  an  upward  movement 
of  the  air,  while  in  an  anticyclone  there  is  a  central  down- 
ward movement. 

Cyclonic  conditions  may  be  characterized  thus :  a  cen- 
tral region  of  low  air  pressure,  with  a  gradual  increase 
from  thence  toward  the  outer  limits ;  a  spiral  motion  of  the 
lower  air  inward  toward  the  center ;  and  in  our  middle  lati- 
tudes, on  the  eastern  and  southern  sides  of  the  center  of 
cyclones,  a  warm,  cloudy,  more  or  less  rainy  condition, 
with  strong  winds  from  the  east  and  south,  and  on  the 


SECONDARY   CIRCULATION   OF  THE  ATMOSPHERE.         21$ 

western  and  northern  sides  a  cool,  clear,  or  clearing  condi- 
tion, with  strong  winds  from  the  west  and  north. 

Anticyclonic  conditions  may  be  characterized  thus :  a 
central  region  of  high  air  pressure,  with  a  gradual  de- 
crease from  thence  to  the  outer  limits ;  a  spiral  motion 
of  the  lower  winds  outward  from  the  center;  and  in  our 
middle  latitudes,  on  the  eastern  and  southern  sides  of  the 
region  of  highest  air  pressure,  a  cool,  clear,  or  clearing 
condition,  with  winds  from  the  north  and  west,  and  on  the 
western  side  a  warm  condition,  with  increasing  cloud,  and 
winds  from  the  south  and  east. 

These  whirls  have  in  our  latitudes  a  motion  of  transla- 
tion of  about  30  miles  per  hour,  usually,  in  an  easterly 
direction ;  and  as  they  pass  over  any  place  lying  in  their 
path,  first  the  conditions  on  the  eastern  or  front  side  are 
experienced,  and  then  those  on  the  western  or  rear  side. 

A ,  cyclone  or  an  anticyclone  may  sometimes  assume 
such  large  proportions  as  to  cover  half  the  United  States, 
and  may  exist  for  several  days,  and  cross  the  entire 
country. 

The  direction  of  the  inward  spiral  movement  of  the  air 
circulating  around  an  extended  cyclone  is,  in  the  northern 
hemisphere,  always  opposite  to  that  of  the  hands  of  a 
watch ;  and  in  the  southern  hemisphere  it  is  with  the 
hands  of  a  watch.  The  direction  of  the  outward  spiral 
movement  of  the  air  circulating  around  an  anticyclone  is 
always  with  that  of  the  hands  of  a  watch  in  the  northern 
hemisphere,  and  opposite  in  the  southern  hemisphere. 

Whether  the  air  is  flowing  down  the  isobaric  surfaces 
inward  towards  the  center  of  low  barometric  pressure  of 
the  cyclone,  or  outward  away  from  the  center  of  high 
barometric  pressure  of  an  anticyclone,  the  rotation  of  the 
earth  on  its  axis  causes  the  current  of  air  to  depart  to 


2l6  ELEMENTARY    METEOROLOGY. 

the  right  in  the  northern  hemisphere,  and  to  the  left  in 
the  southern  hemisphere.  This  gives  the  distinctive  direc- 
tions to  the  whirl  of  the  air  masses  in  cyclones  and  anti- 
cyclones. 

These  cyclones  and  anticyclones  are  sometimes  of  so  great  extent 
and  they  are  so  constantly  undergoing  changes,  that,  in  order  to  study 
such  in  their  true  relations,  it  is  necessary  to  obtain  simultaneous 
observations  of  the  meteorological  elements  over  extensive  regions, 
such  as  continents,  oceans,  or  even  a  whole  hemisphere. 


CYCLONES. 

Classes  of  Cyclones.  —  The  term  cyclone  is  applied  by 
meteorologists  to  all  kinds  of  atmospheric  disturbances  in 
which  the  air  pressure  decreases,  and  there  is  a  wind 
movement,  inward  towards  the  center.  They  are  not, 
however,  all  alike  in  their  other  characteristics. 

There  are  the  cyclones  of  the  torrid  zone,  which  in  their 
greatest  development  are  called  hurricanes  and  typhoons. 
These  atmospheric  whirls  vary  from  a  few  miles  to  several 
hundred  miles  in  diameter,  and  are  characterized  by  great 
violence  of  wind  and  copious  rainfall. 

Cyclones  of  the  temperate  and  colder  zones  are  of 
greater  extent,  usually  covering  a  region  at  least  several 
hundred  miles,  but  sometimes  a  couple  of  thousand  miles, 
in  diameter.  These  are  of  less  intensity  than  the  cyclones 
of  the  torrid  zone,  and  are  the  cyclonic  areas,  or  areas 
of  low  barometer,  usually  spoken  of  in  our  government 
weather  reports. 

The  latter  cyclones  are  frequently  accompanied  by 
secondary  cyclones  of  limited  extent  but  intense  energy, 
manifested  by  strong  winds.  These  are  called  tornadoes. 


SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE.         217 

The  cyclones  of  the  torrid  zone  sometimes  pass  north 
into  the  temperate  zone,  where  they  gradually  lose  their 
former  characteristics  of  relatively  limited  extent  and  great 
energy,  and  spread  out  and  assume  the  main  character- 
istics of  the  cyclones  of  the  temperate  zone. 

Characteristics  of  Cyclones.  —  We  do  not  as  yet  under- 
stand to  a  certainty  the  process  of  formation  of  cyclones, 
and  so  shall  at  first  describe  their  characteristics,  and 
afterward  mention  their  probable  causes. 

The  main  features  of  the  cyclonic  areas  which  must  be 
studied  are  the  distribution  of  air  pressure,  air  tempera- 
ture, moisture,  cloud,  and  rainfall ;  the  direction  and  veloci- 
ties of  air  currents  or  winds ;  and  finally  the  direction  and 
velocity  of  propagation  of  the  cyclonic  areas,  and  the  paths 
usually  pursued  by  them. 

Distribution  of  Air  Pressure  in  Cyclones.  —  In  cyclones, 
at  the  earth's  surface  and  in  the  lower  layers  of  the 
atmosphere  there  is  a  central  area  of  low  air  pressure 
from  which  there  is  an  increase  of  the  pressure  in  all 
directions  (see  within  the  dotted  circle,  Fig.  65).  This  is 
best  represented  by  means  of  isobaric  lines  drawn  for 
equal  intervals  of  barometric  pressure  (after  the  observed 
air  pressures  have  been  reduced  to  seq.  level).  Such  iso- 
baric lines  show  where  the  isobaric  surfaces  intersect 
this  level.  The  isobars  in  the  simplest  ideal  case  are 
concentric  circles,  the  inner  one  of  which  has  the  low- 
est pressure,  the  outer  ones  having  successively  increas- 
ing pressures ;  but  the  isobaric  lines  seldom  present  these 
regular  features,  and  in  nearly  every  case  they  are 
elongated. 

The  ratio  of  the  longer  axis  to  the  shorter  may  be  mqre  than  4 
to  i  ;  on  the  continent  of  America  it  averages  about  2  to  I  ;  while 
on  the  Atlantic  Ocean  the  average  is  only  1.7  to  i. 


218 


ELEMENTARY    METEOROLOGY. 


The  longest  axis  in  our  middle  latitudes  usually  lies  in 
a  direction  from  southwest  to  northeast,  and  its  average 
direction  for  a  great  many  cyclones  in  the  United  States 
is  N.  36°  E.  The  barometric  gradient,  or  rate  of  chang" 


FIG.  65.— AIR  PRESSURE  AND  AIR  MOTION  WITHIN  A  CYCLONIC  AREA  (AFTER  FERREL). 

of  air  pressure,  is  steeper  or  more  rapid,  and  the  isobars 
lie  closer  together,  over  the  ocean  than  over  the  land. 

Where  the  isobars  are  considerably  elongated,  —  that  is, 
are  very  elliptical,  —  there  are  sometimes  two  or  more  dis- 
tinct centers  of  low  pressure  within  the  cyclone ;  but  they 


OF 


SECONDARY  CIRCULATION   OF  THE  ATMOSPHERE.         2IQ 

differ  only  slightly  from  each  other  in  pressure,  and  fre- 
quently have  the  same  pressure. 

The  increase  of  air  pressure  from  the  center  of  cyclones 
to  the  extreme  outer  regions  is  not  a  wholly  uninterrupted 
ascent ;  for  at  some  distance  from  the  center  there  exists  a 
ring  of  higher  air  pressure,  with  anticyclonic  characteristics 
(see  Fig.  65),  which  is  especially  noticeable  in  cyclones  of 
great  energy,  such  as  the  hurricanes  of  the  torrid  zone; 
and  in  cyclones  of  great  extent  it  lies  in  the  region  of  regu- 
lar anticyclones.  This  ring  of  high  pressure  is  similar  to 
that  at  about  latitude  30°  in  the  hemispherical  whirl.  The 
barometric  pressure  at  the  center  of  the  cyclone  sometimes 
descends  as  low  as  27.5  incheSo 

Fig.  68,  a,  on  p.  222,  shows  in  an  ideal  cyclone  the  isobars  at 
sea  level  and  at  an  altitude  of  several  miles. 

Distribution  of  Temperature  in  Cyclones. — The  rear  and 
left-hand  sides,  facing  the  direction  of  translation  (western 
and  northern  sides  of  our  cyclones),  are  the  coldest;  and 
the  front  and  right-hand  (southern  and  eastern)  sides 
are  the  warmest.  But  where  the  winds  blow  from  over 
a  water  surface,  these  conditions  may  be  modified  consid- 
erably, since  the  air  would  then  be  warmer  in  winter  and 
cooler  in  summer  than  that  from  over  a  land  surface. 
In  the  central  and  eastern  United  States  there  is  some- 
times a  difference  of  over  70°  F.  in  the  temperatures 
on  the  warmer  and  colder  sides  of  cyclones;  and  the 
isotherms  which  in  their  normal  condition  run  east  and 
west  will  become  so  changed  as  to  run  north  and 
south. 

The  following  diagram  (Fig.  66)  shows  the  isotherms  in  a  cyclone 
of,  say,  500  miles  in  diameter,  as  indicated  by  the  circle,  along  which 
also  the  direction  of  the  wind  is  showp  by  means  of  arrows* 


220 


ELEMENTARY  METEOROLOGY. 


N 


d   X  f 

FIG.  66.  —  ISOTHERMS  IN  A  CYCLONE.     C°. 
(AFTER  FERREL.) 


With  the  increase  in  the  altitudes  there  is  (at  least  over 
the  interior  of  continents)  a  rapid  decrease  in  the  tempera- 
tures, because  in  the  lower  air  layers  the  temperature 
is  relatively  high,  while  above  it  is  relatively  low.  We 

need  still  more  observations 
on  this  point,  however,  be- 
fore definite  laws  can  be 
formulated. 

Distribution  of  Moisture, 
Cloudiness,  and  Rainfall  in 
Cyclones. —  These  elements 
are  so  intimately  connected 
that  they  may  be  considered 
together.  The  quantity  of 
moisture  in  a  cyclone  depends  so  much  on  its  location 
with  respect  to  the  main  source  of  moisture,  the  sea,  that 
no  general  laws  can  be  enunciated  concerning  it,  or  the 
cloudiness  and  rainfall  re- 
sulting from  it.  In  our 
latitudes  the  eastern,  south- 
eastern, and  southern  sides 
of  the  cyclone  have  the 
greatest  capacity  for  mois- 
ture, because  they  are  the 
warmest ;  and  generally 
the  western  and  northern 
sides  are  the  freest  from 
cloud.  The  various  kinds 
of  cloud  have  such  differ- 
ent origins,  and  lie  at  such 
different  altitudes  above 
the  earth's  surface,  that  their  individual  distribution  in 
the  cyclone  is  difficult  to  give  with  accuracy.  The  cirrus 


>      •      -      >      '      <  Miles. 

0     100    200    300    400    500 

O   Signifies  the  center  of  the  cyclone. 

The  other  lines  show  the  degree  of  cloudiness 

in  percentage  of  an  overcast  sky. 

FIG.  67. — CLOUD  DISTRIBUTION  IN  CYCLONES 
(AFTER  CLAYTON). 


SECONDARY  CIRCULATION   OF  THE  ATMOSPHERE.         221 

clouds,  however,  are  usually  to  be  found  on  the  front  side 
of  a  cyclone,  and  extend  considerably  in  advance  of  it. 

The  relative  humidity  increases  towards  the  center  of  the  cyclone, 
being  greatest  on  the  eastern  and  southeastern  sides. 

The  distribution  of  cloud  around  the  center  of  a  cyclone  in  the 
eastern  United  States  is  shown  in  the  preceding  diagram  (Fig.  67) , 
where  the  top  is  the  northern  side. 

The  Rainfall  occurs  most  frequently  on  the  side  of  the 
cyclone  which  is  nearest  the  supply  of  moisture,  whence 
this  last  is  carried  into  the  region  of  the  cyclone  by  the 
winds  blowing  in  towards  the  center  of  the  cyclone. 

The  great  downpours  of  rain,  however,  occur  at  the 
front  (eastern)  or  warmer  side  of  our  cyclones  two  or 
three  times  as  frequently  as  in  the  cooler  (western)  side. 

The  cyclones  which  are  accompanied  by  excessive  rains 
have  strongly  marked  characteristics,  and  have  very  low 
air  pressures  at  the  center ;  while  those  which  have  light 
rainfall,  or  even  none  at  all,  have  feebler  cyclonic  char- 
acteristics, and  but  slightly  depressed  air  pressures  at  the 
center. 

The  areas  within  which  rain  falls  at  any  one  time  are 
usually  elliptical  in  shape,  and  in  general  are  twice  as  long 
as  they  are  broad ;  and  sometimes  they  are  as  much  as 
2,000  miles  in  length.  It  is  usually  found  that  there  is 
a  central  area  of  maximum  rainfall,  and  this  is  encircled 
by  successive  zones  of  diminishing  amount.  In  the  east- 
ern and  northern  parts  of  the  United  States  the  area  of 
maximum  rainfall  lies  southeast  of  the  center  of  the 
cyclone,  and  usually  at  a  distance  of  about  300  miles 
from  it;  but  the  distance  varies  greatly  in  individual 
instances. 

Cyclones    are    of    more    or    less    gradual   development. 


222  ELEMENTARY   METEOROLOGY. 

The  greatest  depth  of  barometric  pressure  does  not  occur 
at  first,  but  is  reached  later ;  and  the  maximum  amount  of 
rainfall  occurs  at  the  time  of  minimum  pressure. 

Direction  and  Velocities  of  Winds  in  Cyclones.  —  As  the 
air  moves  spirally  inward  towards  the  center  of  low  baro- 
metric pressure,  it  intersects  the  successive  isobaric  lines 
at  certain  angles,  which  vary  not  only  on  the  different 
sides  of  the  cyclone  and  at  different  distances  from  the 
center,  but  also  over  a  land  and  over  a  water  surface. 
Fig.  65  shows  this  circulation  at  the  earth's  surface,  and 


\ 


>•  Direction  of  lower  'winds. 

— Isobars  at  the  average  altitude Direction  of  lower  clouds. 

of  cirrus  clouds.  —  ->  Direction  of  cirrus  clouds. 

— Surface  Isotherms.  QQ      Central  region  of  calms  at  the 

different  altitudes. 

FIG.  68. —  ISOBARS,  ISOTHERMS,  AND  WINDS  AT  VARIOUS  ALTITUDES  IN  A  CYCLONE. 

(Koppen.) 

aloft,  from  the  outer  to  the  inner  limits  of  the  cyclone. 
The  wind  cuts  the  isobars  at  a  smaller  and  smaller  angle 
as  the  center  is  approached;  that  is,  it  turns  away  more 
and  more  from  the  center  as  it  moves  inward,  and  finally 
close  to  the  center  flows  nearly  along  the  isobars. 

The  direction  of  the  wind  circulation  around  a  cyclone  at  various 
altitudes  is  shown  in  the  accompanying  diagram  (Fig.  68,  £),  in  which 
the  arrows  fly  with  the  wind.  The  heavy  unbroken  arrows  show  the 


SECONDARY   CIRCULATION   OF  THE  ATMOSPHERE.         223 

direction  of  air  motion  near  the  ground ;  the  light  unbroken  arrows, 
that  at  the  height  of  the  ordinary  lower  clouds,  at  perhaps  an  altitude 
of  from  6,000  to  9,000  feet ;  and  the  broken  arrows,  that  at  the  height 
of  the  cirrus  clouds,  at  an  altitude  of  perhaps  from  20,000  to  30,000  feet. 

The  arrows  in  Fig.  68,  b,  show  that  in  the  lower  air 
layers  the  direction  of  the  wind  is  towards  the  interior  of 
the  cyclone,  at  a  moderate  altitude  it  is  tangent  to  the 
isobars,  and  at  a  high  altitude  it  is  away  from  the  cyclone. 
This  relation  expressed  more  concisely  is  as  follows :  the 
direction  of  the  air  movement  around  a  cyclone  in  the 
northern  hemisphere  departs  more  and  more  to  the  right 
with  increase  of  altitude. 

The  wind  velocity  increases  from  the  outer  edge  towards 
the  center  of  a  cyclone,  but  the  maximum  velocity  is 
reached  at  some  distance  from  the  center  (several  hundred 
miles  in  some  cases);  and  then  the  velocities  decrease 
again,  sometimes  with  great  suddenness.  Directly  at  the 
center  there  is  usually  a  calm,  or  but  very  little  horizontal 
motion  of  the  air.  The  velocities  at  the  place  of  maximum 
wind  depend  on  the  intensity  of  the  cyclone  and  the  depth 
and  steepness  of  the  gradients,  and  even  near  the  ground 
frequently  reach  40,  50,  or  60  miles  an  hour. 

Magnitude  of  Cyclones.  — The  usual  diameter  of  cyclones 
over  the  United  States  is  from  1,000  to  1,500  miles,  while 
over  the  North  Atlantic  Ocean  it  is  a  few  hundred  miles 
more.  The  barometric  gradients  are  about  15%  steeper 
over  the  North  Atlantic  than  over  the  United  States,  and 
this  is  perhaps  due  to  the  greater  friction  of  the  lower  air 
layers  over  the  land  than  over  a  water  surface.  The  cy- 
clones thus  develop  more  force  over  the  ocean  than  over 
the  continents. 

Direction  and  Velocity  of  Movement  of  Cyclones.  —  Cy- 
clones, in  general,  move  in  the  direction  of  motion  of  the 


60 
EQUATOR 

FIG.  69.  —  PATHS  OF  NUMEROUS  CYCLONES  IN  THE  NORTHERN  HEMISPHERE  (AFTER  LOOMIS). 
(224) 


SECONDARY  CIRCULATION   OF  THE  ATMOSPHERE.         225 

great   mass    of    air  carried   by  the  primary  atmospheric 
currents. 

The  preceding  chart  (Fig.  69)  shows  the  paths  pursued  by  a  great 
many  individual  cyclones.  The  arrow-headed  curved  lines  show  at 
their  beginning  the  place  of  formation,  their  length  shows  the  path 
pursued,  and  the  points  of  the  arrows  show  the  direction  of  motion 
and  place  of  dissipation  of  individual  cyclones. 

The  direction  of  translation  of  cyclones  for  the  middle 
latitudes  is  easterly,  while  for  the  lower  latitudes  it  is  west- 
erly, corresponding  to  the  direction  of  the  air  currents  in 
the  general  atmospheric  circulation  at  those  latitudes. 

The  cyclones  of  our  middle  latitudes  have  an  average 
direction  of  translation  of  about  N.  80°  E.,  or  10°  north 
of  an  easterly  direction ;  but  this  varies  somewhat  for  dif- 
ferent regions.  In  the  central  United  States  the  direction 
is  a  little  south  of  east,  and  in  the  eastern  part  it  is  a  little 
north  of  east. 

The  cyclones  which  are  confined  to  our  tropics  have  a 
direction  of  about  N.  64°  W.  ;  and  the  Asiatic  or  East 
Indian  cyclones  have  a  velocity  of  motion  of  about  9  miles 
per  hour,  while  those  of  America  or  the  West  Indies  move 
about  12  miles  per  hour. 

The  cyclones  which  first  appear  in  the  tropics  with  a  westerly 
motion,  anr3  then  cross  over  into  middle  latitudes  and  move  in  an 
easterly  direction,  have  in  the  West  Indies  at  first  an  average  direc- 
tion of  N.  64°  W.  and  a  velocity  of  17  miles  per  hour,  and  in  middle 
latitudes  a  direction  of  N.  52°  E.  with  a  velocity  of  20  or  21  miles  per 
hour;  while  those  in  the  East  Indies  have  at  first  an  average  direction 
N.  52°  W.  and  a  velocity  of  8  miles  per  hour,  and  later  in  the  middle 
latitudes  a  direction  of  N.  55°  E.  and  a  velocity  of  10  miles  per  hour. 

There  is  a  tendency  for  these  cyclones  to  pursue  some- 
what the  same  tracks,  according  to  the  place  of  origination 


(226) 


SECONDARY   CIRCULATION   OF  THE  ATMOSPHERE.          22 J 

and  the  special  characteristics  of  the  individual  cyclones. 
Sometimes  two  cyclones  coalesce  and  form  a  single  cyclone, 
and  at  other  times  a  single  one  will  divide  up  into  two,  and 
these  will  pursue  quite  distinct  paths. 

The  average  tracks  of  observed  cyclones  over  portions  of  the  north- 
ern hemisphere  are  shown  in  Fig.  70. 

The  velocity  of  translation  of  cyclones  is  nearly  twice 
as  great  in  winter  as  in  summer,  and  seems  to  increase 
up  to  a  middle  latitude,  and  then  decrease  again.  The 
cyclones  of  the  United  States  move  with  an  average 
velocity  of  about  30  miles  per  hour,  which  is  considera- 
bly faster  than  that  of  cyclones  of  the  Atlantic  Ocean 
and  Europe. 

The  Seasons  of  Greatest  Frequency  of  Cyclones. — The 
cyclones  of  the  West  Indies  and  the  China  Sea  occur  most 
frequently  in  July,  August,  September,  and  October ; 
those  of  the  Java  Sea  and  the  South  Indian  Ocean,  in 
December,  January,  February,  March,  and  April  (in  the 
latter  months  for  the  regions  more  distant  from  the  equa- 
tor); those  of  the  Arabian  Sea  and  Bay  of  Bengal,  from 
April  to  June,  and  again  in  October  and  November.  In  the 
middle  northern  latitudes  the  cyclones  occur  most  frequently 
probably  in  February,  and  least  frequently  in  July. 

Region  of  Maximum  Number  of  Cyclones.  —  The  region 
over  which  the  maximum  number  of  cyclone  tracks  have 
been  traced  in  the  northern  hemisphere  lies  along  the  fol- 
lowing course :  commencing  in  the  region  south  of  Japan 
and  Corea,  it  passes  thence  through  Bering  Sea  to  the 
southern  part  of  Alaska ;  thence  along  the  coast  almost 
to  Oregon ;  thence  nearly  due  east  across  the  continent  to 
the  southern  part  of  Newfoundland;  thence  east-northeast 
over  the  Atlantic  Ocean  to  the  Orkney  Islands,  and 


(228) 


SECONDARY   CIRCULATION  OF  THE  ATMOSPHERE,         22Q 

farther  on  to  northern  Norway  and  Sweden ;  thence  south- 
east to  the  interior  of  Russia  and  Siberia,  in  which  dry 
region  they  probably  die  out  for  lack  of  the  moisture  in 
the  air  which  seems  necessary  for  their  long  continuation. 
The  freedom  of  the  region  within  the  Arctic  Circle 
from  cyclones  is  due  partly  to  the  low  temperature  and 
smaller  amount  of  moisture  there,  and  partly  also  to  the 
descending  air  currents  of  that  region,  and  to  the  absence 
of  the  great  horizontal  currents  of  middle  latitudes. 

The  frequency  of  occurrence  and  region  of  maximum  number  of  cy- 
clones in  various  parts  of  the  northern  hemisphere  are  shown  in  Fig.  71, 
where  the  numerals  signify  the  number  of  cyclone  centers  which  have 
passed,  during  a  period  of  10  years,  over  the  regions  along  the  lines. 
The  heavy  arrow  lines  show  the  most  frequented  paths, 

Hurricanes  and  Typhoons.  —  The  hurricanes  of  the  West 
Indies  and  the  typhoons  of  the  East  Indies  are  cyclones 
possessing  both  great  extent  and  power.  Commencing 
usually  between  latitudes  10°  and  20°  N.,  they  pursue 
a  course  a  little  to  the  north  of  west  until  they  reach 
latitude  30° ,  when,  if  they  last  long  enough,  they  swerve 
around  towards  the  east,  and  pursue  a  course  north  of 
east.  They  diminish  in  intensity  with  their  northward 
progress,  and  finally  enter  upon  the  path  frequented  by 
the  cyclones  of  middle  latitudes,  and  disappear  in  the  same 
manner  as  these. 

The  time  which  it  takes  for  a  hurricane  to  pass  over  a  given  place 
lying  within  its  path  varies  from  a  few  hours  to  a  day  or  more,  when 
we  count  the  time  during  which  the  barometer  is  falling  as  the  hurri- 
cane approaches,  and  rising  as  it  recedes  from  the  place.  Hurricanes 
are  never  more  than  a  few  hundred  miles  in  diameter ;  but  they  have 
long  paths,  and  thus  their  force  is  distributed  over  a  large  territory. 

As  the  hurricane  is  approaching,  there  is  a  decrease 
of  pressure,  a  slight  decrease  of  temperature  (during  the 


230  ELEMENTARY  METEOROLOGY. 

western  or  northwestern  progress),  a  rapid  increase  of 
wind  velocity,  and  a  quite  constant  wind  direction.  At 
the  center  there  is  almost  a  calm.  At  the  rear  of  the 
center  there  is  an  increase  of  pressure,  an  increased  tem- 
perature, first  an  increase  and  then  a  decrease  of  the  wind 
velocity;  and  the  wind  blows  quite  constantly  from  a  direc- 
tion nearly  opposite  to  that  on  the  front  side.  The  baro- 
metric pressure  sometimes  falls  two  inches  at  the  center 
of  one  of  these  cyclones. 

One  typhoon  was  traced  from  near  Manilla,  where,  on  Sept.  27,  1882, 
it  had  a  movement  forwards  of  but  5  miles  per  hour,  to  the  coast  of 
Japan  (about  Oct.  i),  where  it  moved  at  the  rate  of  33  miles  per  hour, 
which  increased  to  51  miles  per  hour  on  Oct.  3.  just  east  of  Japan ; 
thence  it  passed  to  the  Aleutian  Islands  and  to  Oregon,  which  it 
reached  on  Oct.  10;  thence  across  the  Rocky  Mountains  at  a  rate  of 
37  miles  per  hour,  and  through  the  northern  United  States  and 
Canada,  Hudson  Bay,  and  Labrador,  to  Davis  Strait;  and  thence 
past  the  southern  point  of  Greenland  to  longitude  27°  west,  latitude 
55°  north,  in  the  Atlantic  Ocean,  where  it  united  with  another  cyclone. 
After  this  union,  the  cyclone  remained  stationary  for  nearly  a  week 
(from  Oct.  19  to  Oct.  24),  when  it  suddenly  took  a  southeast  course 
towards  England,  passed  over  the  Bay  of  Biscay,  and  reached  France 
on  Oct.  27,  took  another  northeasterly  trend,  and  vanished  in  the 
region  of  the  Baltic  Sea  on  Nov.  i,  having  thus  traveled  over  14,000 
geographical  miles  in  35  days. 

Eye  of  the  Storm.  —  Directly  at  the  center  of  the  cy- 
clone there  is  a  region  in  which  the  air  may  be  quite 
free  from  cloudiness,  and  in  which  there  is  but  little  hori- 
zontal air  motion.  This  is  undoubtedly  due  to  the  exist- 
ence of  a  descending  current  at  higher  altitudes  above  the 
ascending  current  at  the  interior  of  the  cyclone  at  low 
altitudes.  The  descending  current  prevents  the  ascend- 
ing current  from  reaching  a  sufficient  altitude  to  cause 
precipitation,  and  perhaps  even  the  formation  of  clouds, 


SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE.          231 

and  tends  to  dissipate  clouds  already  formed.  This  phe- 
nomenon becomes  so  marked,  in  the  case  of  the  limited 
but  intense  tropical  cyclones,  that  the  limits  of  the  clear 
space  at  the  center  can  be  distinctly  seen,  and  it  is  called 
the  "  eye  of  the  storm."  In  the  cyclones  of  higher  latitudes 
it  is  not  so  pronounced  in  character,  but  is  of  wider  extent. 

Secondary  Cyclones  sometimes  occur  within  the  bounda- 
ries of  our  larger  cyclones,  usually  to  the  southeast  of  the 
center  of  the  main  cyclone,  where  the  air  is  warmer  and 
moister  than  in  other  quarters.  They  have  an  air  circulation 
against  the  hands  of  a  watch,  like  the  primary  cyclones. 
Sometimes  they  are  so  slight  as  to  cause  merely  a  small 
distortion  of  the  isobars  of  the  main  cyclone,  with  but 
little  disturbance  of  the  wind  direction;  and  sometimes 
they  cause  a  fairly  well  developed  cyclonic  wind  circula- 
tion. The  local  upward  currents  of  these  secondary  cy- 
clones cause  an  increase  in  the  cloudiness,  and  more  rainfall 
at  the  points  where  they  exist.  Tornadoes  and  kindred 
phenomena  are  closely  allied  to  the  secondary  cyclones, 
and  it  is  scarcely  possible  to  draw  a  dividing  line  between 
them.  It  has  been  deemed  best,  however,  to  treat  of  tor- 
nadoes separately  in  the  next  chapter. 

Origin  of  Cyclones.  —  When  air  flows  along  in  a  current, 
there  is  a  tendency  for  it  to  break  up  into  whirls  or  eddies 
on  the  outer  edges  of  the  current,  and  so,  near  the  borders 
of  the  great  currents  of  the  primary  air  circulation,  such 
vortices  are  formed  (see  Fig.  72);  and  the  centrifugal 
force  resulting  from  this  local  whirling  motion  causes  the 
air  to  recede  from  the  center  of  each  whirl,  and  to  be 
heaped  up  on  the  outer  edge,  making  a  more  or  less  reg- 
ular increase  of  air  pressure  (gradient)  from  the  center 
to  the  outer  edge  of  the  whirl.  These  whirls  might  form 
the  cyclones,  or  areas  of  low  air  pressure ;  and  the  inter- 

WALDO    METEOR.  —  14 


232  ELEMENTARY   METEOROLOGY. 

vening  spaces,  where  the  air  is  heaped  up,  the  anti- 
cyclones, or  areas  of  high  air  pressure.  This  theory, 
however,  has  not  been  worked  out  with  any  degree  of 
completeness. 

g) G) (3)        6)        6) <3) §) 6) g) (3) 

>-        Central      >  Air  y      Current       1-> 

~~Q) Q) Q)        (3)        Q)        Q)        Q) Q)        §> Q) 
FIG.  72.  —  BREAKING-UP  OF  AN  AIR  CURRENT  INTO  WHIRLS. 

Another  Source  of  Cyclonic  Motion  is  to  be  found  in  the 
local  temperature  conditions.  If  anywhere,  as  in  the  inte- 
rior of  the  continent,  a  local  region  becomes  excessively 
heated,  the  air  expands  upward,  and  the  isobaric  surfaces 
become  elevated,  and  cause  an  outflow  of  air  aloft  into  the 
colder  surrounding  air.  The  increased  air  pressure  in  this 
outer  region  causes  the  air  near  the  surface  of  the  ground 
to  flow  inward  toward  the  warm  central  area,  where  the  air 
pressure  has  been  diminished  by  the  overflow  of  air  aloft 
Connecting  these  two  horizontal  currents,  there  is  an  as- 
cending air  current  over  the  central  warm  region,  and  a 
descending  current  on  the  outer  cooler  boundary. 

The  direction  of  these  horizontal  currents,  where  they 
extend  over  hundreds  of  miles,  is  affected  by  the  deflecting 
force  of  the  earth's  rotation.  The  upper  outflowing  cur- 
rent and  the  lower  inflowing  current  are  deflected  towards 
the  right  (in  the  northern  hemisphere);  and  so,  instead  of 
having  a  flow  of  air  directly  from  and  toward  the  heated 
center,  there  results  a  curvilinear  or  spiral  motion  in  both 
currents. 

The  inflowing  surface  current  is  deflected  towards  the  right  of  the 
central  area,  and  flows  spirally  around  it  in  the  direction  opposite  to 


SECONDARY   CIRCULATION  OF  THE  ATMOSPHERE.         233 

that  of  the  hands  of  a  watch  (face  upwards) .  As  this  air  current  ap- 
proaches the  center,  the  velocities  are  much  increased,  because  the  same 
amount  of  air  has  to  pass  in  a  given  time  through  a  much  narrower  space 
nearer  the  center  than  farther  away  from  it.  The  horizontal  motion  be- 
comes more  and  more  changed  to  the  vertical  as  the  immediate  center  is 
approached,  and  near  the  center  becomes  an  upward  current ;  and  this 
upward  current,  too,  may  have  great  velocities. 

The  upper  horizontal  current  flowing  away  from  the  center  is  like- 
wise deflected  towards  the  right,  and  its  spiral  motion  is  in  the  same 
direction  as  that  of  the  hands  of  a  watch  (in  the  northern  hemisphere)  ; 
and  since  the  channel  for  the  air  constantly  increases  with  the  distance 
from  the  center,  the  velocities  of  the  current  decrease,  and  on  the  out- 
skirts of  the  whirl  there  is  but  a  gradual  settling-down  of  the  air,  instead 
of  a  swift  down-moving  current. 

The  lower  inflowing  current  is  much  more  clearly  marked  than  the 
upper  outflowing  current,  and  its  gyratory  motion  is  so  great  that  the 
opposite  gyratory  motion  of  the  upper  current  is  at  first  required  to 
overcome  this  lower  gyratory  motion  before  it  can  assert  itself  in  its 
proper  direction ;  hence,  in  the  neighborhood  of  the  warm  center,  the 
gyration  of  the  whole  air  mass  is  against  the  hands  of  a  watch. 

As  the  upper  outward  current  increases  its  distance  from  the  center, 
the  effect  of  the  deflecting  force  becomes  greater,  and  finally  on  the  out- 
skirts of  the  whirl  overcomes  the  less  developed  deflecting  forces  of  the 
lower  current,  and  imparts  to  the  whole  air  mass  its  distinctive  direction 
of  rotation. 

Therefore,  for  a  cyclonal  region  of  this  kind,  nearly  the  whole  mass 
of  air  at  the  interior  has  a  circulation  around  the  center  in  a  direction 
opposite  to  that  of  the  hands  of  a  watch  ;  while  outside  of  this  there  is  a 
ring  or  zone  in  which  most  of  the  air  has  a  distinctive  rotation  around 
the  same  center,  but  in  the  opposite  direction  to  that  at  the  interior. 
Since  the  friction  with  the  ground  is  such  as  to  make  the  gyratory 
velocity  less  below,  and  the  friction  of  one  air  layer  on  another  is  very 
slight  aloft,  then  the  less  the  altitude  of  the  air  disturbances  above  the 
ground,  the  more  powerful  will  be  the  effect  of  the  upper  air  current, 
and  the  nearer  to  the  center  will  be  the  region  where  the  air  circulation 
changes  from  the  cyclonic  to  the  anticyclonic.  This  is  analogous  to  the 
general  hemispherical  circulation  of  the  atmosphere.  The  directions 
of  these  air  currents  are  shown  in  Fig.  65.  The  full  arrows  indicate  di- 
rections of  lower  currents,  and  the  broken  arrows  those  of  upper  currents. 


234  ELEMENTARY   METEOROLOGY. 

It  is  probable  that  some  cyclones  are  due  entirely  to  the 
breaking-up  into  vortices  of  steady  air  currents,  while 
others  are  due  entirely  to  the  local  unequal  heating  of  the 
air,  and  still  others  are  due  to  a  combination  of  the  two 
processes. 

Moisture  most  probably  plays  a  very  important  part  in 
the  maintenance  of  cyclones,  since  the  large  quantities  of 
heat  liberated  by  condensation  must  retard  the  normal 
cooling  of  ascending  air  very  appreciably,  and  thus  con- 
tinue the  condition  of  instability.  Aside  from  the  fact 
that  local  unequal  heating  of  the  air  may  arise  from  the 
freeing  of  heat  by  condensation  when  upward  currents 
of  moist  air  are  caused  by  outside  influences,  there  is  very 
great  uncertainty  as  regards  the  influence  of  moisture  in 
the  formation  of  cyclones. 

Air  Density.  —  The  local  density  of  the  air  is  dependent  mainly  on 
its  temperature,  and  to  a  slighter  degree  on  its  humidity.  It  has  been 
shown  that  the  lines  connecting  regions  having  air  of  equal  densities 
(over  Europe  at  least)  run  parallel  to  the  coast,  and  that  the  density 
decreases  towards  the  interior  of  the  continent  in  summer,  and  towards 
the  ocean  in  winter.  The  study  of  air  densities  in  connection  with 
cyclones  and  anticyclones  has  not  been  thoroughly  carried  out ;  but  it 
has  been  stated  that  when  the  closeness  of  the  lines  of  equal  density 
signify  an  unstable  condition  of  equilibrium,  then  cyclonic  disturbances 
tend  to  form,  and  they  move  along  parallel  to  the  lines  of  equal  density. 
It  is  very  probable  that  the  accurate  and  careful  study  of  the  densities 
of  the  air  will  lead  to  a  better  knowledge  of  the  formation  and  behavior 
of  secondary  or  local  cyclonic  disturbances. 


ANTICYCLONES. 

Classes  of  Anticyclones.  —  There  are  several  classes  of 
areas  of  high  barometric  pressure.  One  class  is  due  to  the 
heaping-up  of  the  air  thrown  off  by  centrifugal  force  from 


SECONDARY  CIRCULATION   OF  THE   ATMOSPHERE.         235 

the  cyclonal  whirls.  Such  is  the  ring  of  high  pressure 
which  encircles  the  globe  at  about  latitude  30°.  When 
cyclonal  areas  enter  this  ring  of  high  pressure  from  with- 
out, the  air  is  still  further  heaped  up  on  their  outskirts ; 
and  these  masses  of  air  are  forced  out  from  the  main  ring 
especially  on  its  poleward  side,  and  are  made  to  project  far 
into  the  higher  latitudes  between  the  successive  cyclones. 

Another  class  of  high-pressure  areas  is  due  to  local 
cooling  of  the  air.  These  are  the  isolated  areas  of  high 
barometric  pressure,  which  are  essentially  phenomena 
of  the  colder  and  continental  latitudes,  where  the  lower 
air  is  cold  and  dense.  The  isobaric  surfaces  being  thus 
brought  close  together  at  the  cold  center,  the  upper  air 
flows  down  them  towards  the  center  from  the  warmer 
regions  without,  and  increases  the  pressure  at  the  center. 

Cyclones  and  Anticyclones  contrasted.  —  Anticyclones 
are  by  no  means  so  well  marked  in  their  limits  and  char- 
acteristics as  cyclones.  In  anticyclones  the  air  pressure  is 
greatest  at  the  center,  and  decreases  thence  in  all  direc- 
tions somewhat  irregularly ;  and  on  the  side  where  a 
cyclone  is  to  be  found,  this  decrease  continues  practi- 
cally uninterruptedly  to  the  region  of  lowest  air  pressure 
in  the  cyclone. 

Anticyclones  are  usually  an  accompaniment  of  cyclones ; 
and  in  fact  where  a  cyclone  exists,  with  its  deficiency  of  air, 
there  must  lie  at  not  a  great  distance  trom  it  one  or  more 
anticyclonic  areas  into  which  the  air  has  overflowed  from 
the  upper  region  of  the  cyclone.  Sometimes  the  cyclones 
are  surrounded  by  a  ring  of  anticyclones,  but  the  continuity 
of  the  latter  is  nearly  always  broken  by  slight  depressions 
or  troughs  in  which  the  air  pressure  is  lower  than  the 
maximum  barometric  pressure  in  the  anticyclones,  which 
in  most  cases  is  above  30  inches. 


236  ELEMENTARY   METEOROLOGY, 

Air  Pressure  in  Anticyclones,  —  In  ordinary  anticyclones 
of  our  middle  latitudes  the  barometric  pressure  frequently 
reaches  30.6  inches,  and  in  some  cases  it  exceeds  even  31 
inches.  The  isobars  are  even  more  irregular  than  in  cy- 
clones, and  are  usually  elongated  (elliptical),  with  the  great- 
est axis  pointing  most  frequently  about  northeast  and  south- 
west, although  it  may  be  directed  towards  any  point  of  the 
compass.  The  longest  diameter  is  on  the  average  about 
twice  the  length  of  the  shorter,  but  sometimes  exceeds  it 
fourfold.  The  relative  length  and  direction  of  the  axes  de- 
pend a  good  deal  on  the  adjacent  areas  of  low  barometer. 
When  an  anticyclone  is  situated  between  two  well-devel- 
oped cyclones  not  far  apart,  then  the  anticyclone  becomes 
much  elongated;  and  when  this  latter  condition  obtains 
there  are  sometimes  two  or  three  distinct  centers,  which 
differ  in  maximum  pressure  by  only  slight  amounts. 

The  dimensions  of  anticyclones  exceed  those  of  cyclones.  It  has 
been  found  that  the  diameter  of  those  in  the  winter  time,  with  extraor- 
dinarily high  pressures,  was  from  3,000  to  4,000  miles ;  and  the  pre- 
ceding and  following  cyclone  centers  were  distant  fully  2,000  miles 
from  the  center  of  the  anticyclone,  and  the  preceding  cyclone  was  much 
the  deeper  of  the  two,  and  the  steepest  gradients  were  on  this  side. 
Such  relations  are  shown  in  Fig.  74. 

Moisture  in  Anticyclones. — Anticyclones  are  character- 
ized by  a  dry  cool  air  and  little  cloud,  and  but  slight  precip- 
itation. The  descending  air  at  the  center  tends  to  dissipate 
the  cloudiness.  This  is  due  to  the  fact  that  as  the  air 
descends  it  is  subject  to  a  greater  compression,  becomes 
denser,  and  the  temperature  is  raised ;  by  which  means 
the  relative  humidity  is  decreased,  and  cloud  is  dissipated. 
Ground  fog  frequently  occurs  in  anticyclones,  because  on 
unclouded  nights  just  at  and  very  close  to  the  ground  the 


SECONDARY  CIRCULATION   OF  THE  ATMOSPHERE.       237 

temperature  abruptly  decreases  to  below  the  point  of  con- 
densation, and  fog  is  formed. 

Temperature  Distribution  in  Anticyclones.  —  Near  the 
earth's  surface  the  temperatures  are  generally  below  the 
normal  air  temperatures  ;  and  usually  the  more  pronounced 
the  anticyclone  (i.e.,  the  greater  the  air  pressure),  the 
lower  the  temperature.  At  higher  altitudes,  however,  the 
air  is  but  little  colder  than  near  the  ground,  and  in  many 
cases  there  is  even  an  inversion  of  the  usual  temperature 
decrease  with  altitude,  and  it  is  actually  warmer  above  than 
below.  This  is  due  to  the  excessive  radiation  of  heat  from 
the  ground  into  the  clear  sky.  In  the  anticyclones  of  our 
middle  latitudes  the  air  is  colder  on  the  eastern  side. 

Direction  and  Velocity  of  the  Wind  in  Anticyclones.  - 
Near  the  surface  of  the  ground  in  anticyclones  the  air 
motion  is  directed  spirally  outward.  The  winds  at  high 
altitudes  are,  on  the  contrary,  directed  spirally  inward  ;  and 
at  the  center  of  high  pressure  there  is  a  vertical  downward 
current  connecting  the  inflow  above  with  the  outflow  below. 

The  wind  velocities  in  anticyclones  are  much  smaller 
than  those  in  cyclones.  The  inner  downward  current  has 
probably  very  much  less  velocity  than  the  upward  current 
in  cyclones,  and  usually  amounts  to  but  little  more  than  a 
settling-down  of  the  air  towards  the  earth's  surface.  Calms 
are  very  frequent  within  anticyclones. 

The  Direction  and  Velocity  of  Propagation  of  Anticyclones. 
—  Anticyclones,  like  cyclones,  drift  along  in  the  general 
direction  of  the  great  air  currents.  Since  they  occur 
mostly  in  the  middle  and  higher  latitudes,  their  direction 
is  generally  easterly0  In  the  United  States  the  anticy- 
clones move,  on  the  average,  in  a  southeasterly  direction, 
that  is,  at  right  angles  to  the  direction  of  greatest  elon- 
gation; but  in  winter  the  direction  is  a  little  more  east- 


238  ELEMENTARY   METEOROLOGY. 

erly.  The  velocity  of  translation  is  not  so  great  as  that  for 
cyclones,  nor  are  the  paths  pursued  so  long.  The  reason 
of  this  is  not  well  understood,  but  it  is  supposed  that  some 
process  in  the  formation  or  continuance  of  cyclones  accel- 
erates their  movement  in  certain  directions ;  and  this  fea- 
ture is  lacking  in  the  anticyclones,  which  lag  behind. 

Excessive  High  Pressure  in  Anticyclones  -  -  In  the 
case  of  an  ordinary  anticyclone  at  the  rear  of  a  cyclone, 
we  have  very  little  cloud,  and  consequently  the  outward 
radiation  is  very  rapid  and  great,  and  the  ground  and 
lower  air  layers  are  still  further  cooled.  This  causes  the 
lower  air  to  become  still  denser,  and  the  isobaric  surfaces 
are  lowered  so  that  the  air  at  greater  altitudes  in  the  sur- 
rounding region  flows  down  these  surfaces  and  in  towards 
the  center.  This  increases  the  air  pressure,  which  was 
already  high,  and  causes  it  to  become  very  high  if  the 
air  becomes  sufficiently  cooled.  This  accounts  for  the 
extremely  high  air  pressures  observed  in  winter. 

In  considering  both  cyclones  and  anticyclones,  it  must 
be  remembered  that  their  lateral  extent  is  perhaps  a  thou- 
sand times  their  thickness,  and  the  mass  of  gyrating  air 
may  be  regarded  as  a  relatively  thin  disk.  There  is  thus 
a  chance  for  a  great  waste  of  force  through  friction,  and 
innumerable  opportunities  for  the  interference  with  the 
horizontal  circulation  by  means  of  vertical  currents.  The 
original  forces  which  give  rise  to  these  phenomena  can 
thus  soon  become  used  up. 

Relation  of  Cyclones  to  Anticyclones.  —  Anticyclones  are 
a  natural  consequence  of  cyclones,  because,  when  there 
is  a  diminution  of  pressure  in  one  place  (in  the  cyclone), 
there  must  be  a  corresponding  increase  in  another  place 
(in  the  anticyclone).  The  air  movement  between  a  cyclone 
and  an  anticyclone  is  shown  in  the  accompanying  diagram 


Lower  current 
Upper  current 


SECONDARY   CIRCULATION   OF  THE   ATMOSPHERE.         239 

(Fig.  73),  where,  for  the  lower  layers  of  the  air,  the  baro- 
metric minimum  or  cyclone  is  shown  on  the  left,  and  the 
barometric  maximum  or  anticyclone  on  the  right;  and 
the  wind  directions  there  are  shown  by  the  full-drawn 
arrows.  At  higher  alti- 
tudes, however,  in  the 
cloud  region,  the  wind  di- 
rection for  which  is  shown 
by  the  broken-lined  arrows, 
above  the  cyclone  the  di- 
rection of  motion  of  the 

.  .         ,        FIG.  73.  —  INTERCHANGE  OF  AIR  BETWEEN  CY- 

air  IS   Seen  tO    be   antlCyClO-      CLONE  AND  ANTICYCLONE  (NORTHERN  HEMI- 

nal  and  away  from  the  cen-     SPHERE). 

ter,  while  above  the  anticyclone  it  is  cyclonal  and  towards 
the  center.  Thus  the  upper  current  from  the  cyclone  re- 
plenishes the  air  lost  from  the  anticyclone  at  the  surface, 
and  the  lower  current  from  the  anticyclone  replaces  the 
air  lost  from  above  the  cyclone. 

The  cyclones  coming  from  the  tropics  have  to  break 
through  the  ring  of  high  pressure  at  about  latitude  30°  ; 
but  there  the  absolute  mass  of  air  is  so  great,  and  the 
cyclone  so  limited  in  extent,  that  the  air  thrown  out  by 
them  does  not  cause  much  of  an  increase  in  the  pressure ; 
so  that,  while  there  are  well-marked  cyclones  in  the  lower 
latitudes,  yet  the  anticyclones  with  excessively  high  pres- 
sure are  not  to  be  found  there. 


The  simultaneous  distribution  of  cyclones  and  anticyclones  over  the 
northern  hemisphere  is  shown  in  Fig.  74,  which  gives  the  isobars  on 
Dec.  15,  1882.  This  shows  an  area  of  high  pressure  (an  anticyclone) 
over  North  America,  and  one  over  Asia,  with  areas  of  low  pressure 
(cyclones)  over  the  eastern  Pacific  Ocean  and  the  North  Atlantic 
Ocean.  A  less  pronounced  anticyclone  is  also  to  be  seen  over  the 
southern  part  of  the  North  Atlantic  Ocean. 


240 


ELEMENTARY   METEOROLOGY. 


110  100  90  80 


FIG.  74.— CHART  SHOWING  ISOBARS,  DEC.  15, 1882  (AFTER  LOOMIS). 

Permanent  Cyclones  and  Anticyclones.  —  In  the  charts 
showing  the  average  air  pressures  over  the  globe,  it  is 
seen  that  there  are  local  regions  of  high  and  of  low  pres- 
sure,—  the  so-called  permanent  anticyclones  and  cyclones. 


CHAPTER   X. 
LOCAL  AND  MISCELLANEOUS   WINDS. 

Irregular  Local  Atmospheric  Disturbances.  —  In  those 
regions  where  there  arise  disturbances  of  the  condition  of 
equilibrium  of  the  atmosphere  over  a  limited  area,  there  is 
found  a  class  of  phenomena  of  which  the  thunder  squalls, 
spout  phenomena  (such  as  waterspouts,  sand  spouts,  and 
tornadoes),  and  the  straight  blows  (dereckos)  form  the 
principal  members. 

These  are  all  somewhat  closely  related  as  to  their  cause ;  and  the 
final  development  of  one  form  or  another  is  due  to  variations  in  the 
conditions  under  which  they  arise,  and  somewhat  also  to  the  geograph- 
ical locations.  While  these  phenomena  are  individually  of  but  small 
extent,  yet  the  conditions  favorable  to  their  formation  may  extend 
over  a  considerable  area,  so  thai,  instead  of  individuals,  groups  may 
exist  at  any  one  time,  and  these  progress  in  parallel  tracks. 

TORNADOES. 

A  Tornado  is  a  progressive,  limited,  local,  violent  whirl- 
wind, characterized  by  a  funnel-like  cloud  which  hangs 
suspended  from  an  intensely  black  mass  of  storm  clouds ; 
the  apex  of  the  funnel  cloud  sweeps  over  the  earth's  sur- 
face, sometimes  touching  it,  and  sometimes  receding  from 
it,  to  come  down  again  to  the  ground  farther  on  in  the  course 
of  the  cloud  as  it  moves  forward.  An  ideal  picture  of  a 
tornado  in  the  United  States,  as  viewed  when  looking  toward 
the  east,  is  shown  in  Fig.  75.  The  peculiar  funnel-like 
cloud  may  be  seen  at  the  center,  receding  in  a  northeasterly 
direction  from  the  scene  of  destruction. 

241 


242 


ELEMENTARY    METEOROLOGY, 


The  first  visible  precursor  of  one  of  these  tornadoes  is  a 
heavy  bank  of  clouds  appearing  in  the  southwest,  and  later 
in  the  west  and  northwest ;  in  these  a  violent  commotion  is 
observable,  with  adjacent  clouds  rushing  in  from  the  south- 
east, east,  and  northeast,  towards  the  center  of  the  disturb- 
ance. If  the  clouds  are  light  in  color,  they  resemble  smoke 
clouds  from  a  large  fire ;  if  they  are  dark,  they  are  of  a 
peculiar  greenish  hue,  which  increases  in  intensity  as  the 


FIG.  75.  —  PASSAGE  OF  A  TORNADO  (THE  OBSERVER  FACING  THE  EAST). 

clouds  approach.  Sometimes  these  dark  clouds  look  like 
the  dense  volumes  of  smoke  emitted  from  the  smokestack 
of  an  engine.  It  is  among  such  clouds  that  the  first  ap- 
pearance of  the  tornado  funnel  cloud  is  to  be  noticed.  It 
then  seems  to  descend  toward  the  earth  by  gradual  length- 
ening or  growth  of  the  funnel.  The  near  approach  of 
the  cloud  is  announced  by  a  heavy  rumbling  sound  like  that 
of  an  approaching  railroad  train,  or  of  distant  thunder. 


LOCAL   AND    MISCELLANEOUS   WINDS.  243 

The  noise  increases  as  the  cloud  draws  nearer,  and  its 
passage  creates  such  an  uproar  that  all  other  sounds  are 
deadened.  The  observer* in  front  of  this  rapidly  moving 
mass  of  clouds  finds  himself  in  the  region  of  a  gentle  south 
breeze  of  warm  air,  or  of  calm  air  and  oppressive  heat.  This 
quiet  condition  is  suddenly  disturbed  by  the  tempestuous 
whirlwind  caused  by  the  tornado  itself,  which  cuts  a  swath 
in  its  narrow  track,  felling  everything  before  it  in  an  irresist- 
ible manner.  The  passage  of  the  funnel-shaped  cloud  is 
succeeded  by  a  sudden  lowering  of  the  temperature,  and 
by  a  calm  or  gentle  breeze. 

Air  Circulation  in  Tornadoes. — There  is  a  horizontal 
current  of  warm  moist  air  flowing  in  from  all  sides  (but  not 
from  a  great  distance)  towards  a  center,  and  an  upward  cur- 
rent at  this  center  which  is  fed  by  the  horizontal  current 
just  mentioned.  Above  these  there  is  an  outflow  away  from 
the  center.  The  motion  of  the  upward  current  is  not  di- 
rectly upwards,  but  takes  a  spiral  form ;  and  its  position 
is  shown  by  the  outlines  of  the  peculiar  tornado  cloud 
which  hangs  in  funnel  shape,  suspended  from  the  intensely 
black  mass  of  storm  clouds  (see  Fig.  75).  The  direction 
of  this  spiral  rotation  has  been  observed  to  be  opposite 
to  that  of  the  hands  of  a  watch,  in  the  cases  where  this 
phase  has  been  noticed,  but  it  has  not  been  possible 
to  obtain  frequent  careful  observations  of  it. 

Formation  of  Tornadoes.  —  Tornadoes  are  caused  by 
local  differences  of  temperature.  The  air  having  become 
abnormally  heated  over  a  central  area,  there  results  a  differ- 
ence in  pressure  between  the  air  of  the  inner  region  and 
that  surrounding  it ;  from  this  there  arises  a  flow  of  air 
spirally  inward  towards  the  center,  and  as  it  is  approached, 
the  velocity  is  increased.  The  principal  condition  for  the 
formation  of  a  tornado  is  the  local  unstable  condition  of 


244 


ELEMENTARY   METEOROLOGY. 


the  air,  due  to  the  abnormal  heating  of  a  mass  of  air 
either  at  the  earth's  surface  or  at  some  locality  above 
it.  This  mass  of  air,  being  warmer  than  the  surrounding 
air  at  the  same  level,  is  in  unstable  equilibrium ;  and  when 
some  slight  disturbance  frees  it  from  its  abnormal  posi- 
tion, it  is  forced  upwards,  by  the  pressure  of  the  air 
below  and  around  it,  to  that  altitude  where  its  tempera- 
ture and  density  are  normal  (that  is,  the  same  as  the 
temperature  and  density  of  the  surrounding  air);  there 
the  power  causing  the  vertical  current  ceases. 


1300? 


r — i  8-6°  6 


In  this  ascent  the  air  cools  by  expansion ;  but  in  ascending  moist  air, 
when  condensation  occurs  the  cooling  is  slower,  and  consequently  the 
difference  in  temperature  between  the  out- 
side and  inside  column  of  air  becomes 
greater  (Fig.  76),  and  the  upward  velocity  is 
increased.  But  this  ascending  air  at  first 
accumulates  above,  and  the  pressure  thereby 
increases,  so  that  at  some  altitude  the  air 
pressure  over  the  ascending  column  becomes 
greater  than  in  the  surrounding  air.  The 
difference  between  the  interior  and  exterior 
temperature  increases  up  to  this  level ;  but 
above  this  point  it  decreases  (owing  to  the 
greater  density  of  the  air  above  it),  and  at 
some  higher  altitude  the  temperature  is  the 
same  without  and  within  the  current,  and 
still  above  this  (and  also  below  the  altitude 
of  condensation)  the  decrease  of  temperature 
with  the  ascent  is  more  rapid  within  than 
outside.  At  the  altitude  where  the  tern 
perature  becomes  the  same  inside  and 
outside  of  the  current,  the  air  pressure 
also  becomes  the  same.  Below  this  level 
the  temperature  is  greater  and  the  pres- 
sure less  in  the  ascending  current  with  condensation  than  outside  of  it, 
and  above  this  level  the  condition  is  reversed. 


Adjacent 

Space 
temperatures 


Altitude     Central 
Space 

Altitudes  in  meters 
inC.° 

FIG.  76.  —  VERTICAL  DECREASE  OF 
TEMPERATURE  (SPRUNG). 


LOCAL  AND  MISCELLANEOUS  WINDS.       245 

This  increased  pressure  above  forces  the  air  out  on  all  sides  as  it 
moves  upward,  and  this  air  flowing  over  into  the  adjacent  air  mass 
makes  it  heavier,  and  the  air  below  is  forced  in  from  it  to  replace  the 
air  which  has  moved  upwards.  There  is  thus  caused  a  vertical  air 
circulation  upwards  within  the  central  area,  and  downwards  on  all  sides 
around  it ;  while  an  outward  horizontal  current  above,  and  an  inward 
horizontal  current  below,  connect  the  two  vertical  currents.  This  cir- 
culation may  take  place  when  the  unstable  condition  exists  for  any  air 
mass,  whether  this  condition  be  near  the  earth  or  at  high  altitudes,  or 
whether  it  extend  through  the  whole  vertical  air  column  or  not.  If  this 
circulation  took  place  without  any  rotary  motion,  the  velocities  would 
be  small,  and  the  flow  of  air  gentle ;  but  because  of  rotation  the  veloc- 
ities increase,  and  near  the  center  become  very  great,  according  to  the 
law  of  conservation  of  areas.  This  rapid  gyratory  motion  near  the 
center  gives  the  tornado  winds  their  tremendous  velocities. 

With  the  increasing  wind  velocities  towards  the  center  there  is 
an  increasing  centrifugal  force,  and  the  air  is  forced  away  from  the 
center  as  in  the  case  of  cyclones,  and  thus  gradients  arise  in  the  air 
pressure ;  and  these  gradients  are  steeper  the  nearer  the  center  is  ap- 
proached, so  that  there  is  a  decrease  in  the  air  pressure  towards  the 
center  of  the  whirl,  and  near  the  center  this  decrease  becomes  rela- 
tively very  great  in  the  case  of  the  great  rotational  velocities  met  with 
in  tornadoes. 

Sustaining  of  Tornadic  Action.  —  The  difference  in  tem- 
perature within  and  without  the  central  upward  current 
of  the  tornado,  which  is  necessary  for  its  continuance, 
could  not  persist  long  enough  for  its  full  development  if 
it  were  not  for  the  freeing  of  latent  heat  through  conden- 
sation. This  latent  heat  becomes  sensible,  and  retards 
the  cooling  of  the  inner  air  column.  It  is  therefore  when 
the  air  is  saturated  with  moisture  that  the  conditions  are 
best  for  the  maturing  of  a  tornado ;  and  when  conden- 
sation no  longer  takes  place,  the  tornado  action  must 
soon  cease. 

When  and  Where  Tornadoes  Occur.  —  The  conditions 
necessary  for  the  formation  of  tornadoes  are  to  be  met 


346  ELEMENTARY   METEOROLOGY. 

with  at  low  altitudes  in  the  moist  atmosphere  of  the  lower 
and  middle  latitudes.  They  occur  in  western  Africa,1 
southern  Asia,  occasionally  in  the  lower  middle  latitudes 
of  Europe,  but  most  frequently  in  the  central  and  eastern 
portions  of  the  United  States.  Most  of  our  knowledge  of 
these  phenomena  is  obtained  from  the  study  of  those  occur- 
ring in  the  last-named  country,  and  the  following  remarks 
apply  to  them.  The  true  tornado  is  probably  not  observed 
above  the  5Oth  parallel  of  latitude.  Tornadoes  in  the 
United  States  form  most  frequently  several  hundred  miles 
(300  to  500)  to  the  southeast  of  the  center  of  a  cyclone 
which  has  a  long  troughlike  central  depression  extend- 
ing north  and  south  or  northeast  and  southwest.  This 
region  is  the  one  of  greatest  heat  and  moisture,  and  of 
surface  winds  of  considerable  force  from  the  south.  The 
regions  of  greatest  frequency  of  tornadoes  are  the  lower 
Missouri,  the  central  Mississippi,  and  Ohio  river  valleys ; 
and  the  season  of  their  greatest  frequency  is  the  late 
spring  and  summer  for  these  regions,  but  farther  south 
(in  Georgia,  for  instance)  many  tornadoes  occur  in  the 
early  spring.  Few  tornadoes  occur  in  the  United  States 
west  of  the  looth  meridian.  The  hours  of  greatest  fre- 
quency of  tornadoes  are  from  3.30  to  5  P.M. 

Paths  of  Tornadoes.  —  The  path  of  destruction  which 
marks  the  central  region  of  a  tornado  varies  from  a  few 
feet  up  to  2  miles,  the  average  being  about  a  quarter  of  a 
mile.  The  length  of  such  tracks,  which  mark  the  places 
where  the  tornadoes  reached  to  the  earth's  surface,  vary 
from  300  yards  to,  200  miles,  with  an  average  of  about  25 
miles.  The  funnel-shaped  cloud  at  the  earth's  surface 

1  Where  they  occur  in  the  southern  hemisphere,  the  directions  of  motion 
are  the  reverse  of  those  in  the  northern  hemisphere,  because  in  the  former 
the  cyclonal  rotation  is  with  the  hands  of  a  watch. 


LOCAL  AND   MISCELLANEOUS  WINDS.  247 

varies  in  diameter  from  a  few  yards  to  an  unmeasured 
limit,  but  the  average  is  probably  several  hundred  feet. 

Velocity  of  Motion  of  Translation  of  Tornadoes.  —  The 
velocity  of  progression  of  tornadoes  varies  from  7  to  100 
miles  per  hour,  with  an  average  of  about  44  miles  per 
hour.  The  same  tornado  cloud  may  sometimes  remain 
almost  stationary  for  a  while,  and  then  dart  forwards  with 
great  velocity.  The  average  time  occupied  in  passing  a 
point  is  a  little  over  a  minute. 

The  direction  of  motion  of  translation  of  the  tornado  is 
nearly  always  from  southwest  to  northeast. 

Air  Pressures  in  Tornadoes.  —  The  normal  air  pressure 
is  about  14.7  pounds  per  square  inch;  and  if,  as  is  possi- 
ble, the  air  pressure  is  reduced  one  fourth  of  this  amount 
at  the  center  of  a  tornado,  then  it  is  lessened  there  about 
3.7  pounds  per  square  inch,  or  533  pounds  per  square  foot. 
Supposing,  now,  that  the  tornado  passes  over  a  house  in 
which  the  air  is  at  the  normal  pressure  (that  is,  on  the 
inner  and  outer  walls  there  is  a  pressure  of  2,117  pounds 
per  square  foot),  then  the  pressure  on  the  outer  walls 
during  the  passage  of  the  center  of  the  tornado  will  be 
suddenly  diminished  by  533  pounds  per  square  foot,  and 
there  will  be  an  excess  of  pressure  by  this  amount  from 
within  outward.  The  diminution  of  the  outside  pressure 
comes  very  suddenly,  and  this  inside  pressure  acts  like  an 
explosion.  It  is  not  to  be  wondered  at,  then,  that  doors 
and  windows  are  blown  out,  and  walls  even  torn  down, 
when  this  great  force  so  acts  suddenly  on  them  in  an  out- 
ward direction. 

Destructive  Winds  in  Tornadoes.  —  The  enormous  wind 
velocities  in  tornadoes  are  measured  only  indirectly  by 
the  mass  or  strength  of  objects  which  they  have  moved 
or  broken.  It  is  probable  that  they  reach  or  even  ex- 

WALDO  METEOR. —  15 


248  ELEMENTARY   METEOROLOGY. 

ceed  400  or  500  miles  per  hour  in  some  cases.  The  winds 
are  greatest  near  the  center,  and  decrease  from  thence  out- 
wards ;  they  are  less  in  front  than  at  the  rear.  Tornadoes 
are  usually  accompanied  by  hail  fall,  and  very  frequently 
by  manifestations  of  atmospheric  electricity. 

On  the  right  side  of  the  tornado  the  winds  are  stronger 
than  on  the  left-hand  side  of  its  direction  of  progression ; 
and  the  path  of  destruction  extends  much  farther  from  the 
center  on  the  right  than  on  the  left  side. 

The  winds  on  the  left  (or  northwest)  side  are  the  weakest,  because, 
for  one  thing,  the  direction  of  progression  is  there  directly  opposed  to 

^_ that  of  the   gyratory   motion. 

^_ >^  In    Fig.    77    the    long    arrow 

^.    shows    the    direction   of  pro- 

" ^      ^  gression,  and  the  small  arrows 

>^-— — >  the  gyratory   motion  ;   and   it 

FlG<  77'  is  seen  that  on  the  right-hand 

side  the  two  motions  are  in  the  same  general  direction,  while  on  the 
left  the  directions  are  opposed  to  each  other. 

The  wind  force  in  tornadoes  depends  directly  on  the  velocity  of  the 
air  current  and  the  density  of  the  air.  It  has  been  computed,  that,  in 
the  case  of  a  tornado  in  which  the  gyratory  velocity  was  10  feet  per  sec- 
ond at  a  distance  of  3,300  feet  from  the  center,  the  velocity  at  70  feet 
from  the  center  would  be  460  feet  per  second,  or  310  miles  an  hour, 
which  would  give  a  pressure  force  of  about  300  pounds  per  square  foot 
on  any  plane  surface  exposed  squarely  to  the  wind.  In  case  the  ascend- 
ing current  in  a  tornado  had  a  velocity  of  100  miles  an  hour  at  the  alti- 
tude where  the  air  pressure  is  15  inches,  this  would  prevent  a  hailstone 
2.5  inches  in  diameter  from  falling  to  the  ground.  Large  raindrops  o.l 
of  an  inch  in  diameter  require  an  upward  current  of  only  16.5  miles  an 
hour  to  sustain  them  when  the  air  pressure  is  23  inches ;  while  small 
drops  only  0.003  mc^  in  diameter  require  an  air  current  of  only  3  miles 
an  hour  to  prevent  their  falling  to  the  ground. 

The  size  of  raindrops  may  be  taken  as  indicating  roughly  the  veloc- 
ity of  the  ascending  air  current.  When  the  raindrops  are  fine,  there 
is  a  weak  upward  current ;  but  when  they  are  coarse,  there  is  a  more 
powerful  one. 


LOCAL  AND    MISCELLANEOUS   WINDS.  249 

Safety  in  Tornadoes.  —  Concerning  the  safety  of  those 
who  find  themselves  in  the  path  of  a  tornado,  it  may  be 
said  that  it  is  best  to  enter  some  subterranean  vault,  such 
as  an  ice  cellar.  The  cellar  of  a  house  is  preferable  to  the 
house  itself  if  the  house  is  of  wood ;  but  if  it  is  of  stone  or 
brick,  one  should  not  enter  the  cellar,  but  leave  the  build- 
ing. The  southwest  corner  of  a  cellar  is  the  safest. 

If  one  must  escape  a  tornado  in  an  open  place,  he  should 
first  take  the  bearings  of  the  approaching  tornado  and  its 
course.  If  the  tornado  is  discovered  to  the  west  or  south- 
west of  him,  he  should  move  off  with  dispatch  towards 
the  southeast  or  northwest,  respectively.  If  the  tornado 
is  far  to  the  south  or  to  the  north,  one  is  usually  safe. 
If  one  is  caught  in  the  whirlwind's  blast,  it  is  best  to  lie 
prone  on  the  ground  in  an  open  field  rather  than  seek 
shelter  in  a  wood  or  under  trees. 

Frequency  of  Tornadoes.  —  During  the  10  years  from 
1877  to  1887  the  number  of  tornadoes  reported  in  the 
United  States  averaged  146  a  year.  Observations  do  not 
as  yet  prove  that  tornadoes  are  increasing  or  decreasing  in 
frequency. 

Multiple  Tornadoes.  —  Frequently  on  the  same  afternoon 
a  number  of  tornadoes  will  form.  They  may  be  close  to- 
gether or  several  hundred  miles  apart,  and  they  move  in 
parallel  directions.  Thus  they  form  a  group  or  band  of 
isolated  whirlwinds,  all  of  which  have  for  their  origins  the 
same  general  conditions. 


THUNDERSTORMS. 

Thunderstorms  and  their  Attendant  Phenomena.  —  Thun- 
derstorms are  local  progressive  atmospheric  disturbances 
occurring  in  most  latitudes  inhabited  by  mankind,  in 


ELEMENTARY   METEOROLOGY. 

regions  where  there  is  considerable  atmospheric  moisture. 
They  receive  their  name  from  the  electrical  phenomena 
which  distinguish  them.  They  are  usually  accompanied 
by  rainfall.  Their  attendant  phenomena  in  our  latitudes 
are  as  follows  :  First  dark  clouds  are  seen  lying  low  in  the 
western  sky,  and  light  southern  winds  are  experienced, 
the  air  being  warm  and  sultry.  Perhaps  an  hour  later  the 
clouds  have  mounted  to  near  the  zenith,  but  the  same  rel- 
ative quietness  of  the  air  continues  near  the  ground ;  and 
the  heat  is  perhaps  not  so  great,  owing  to  the  sheltering 
influence  of  the  clouds.  The  latter  usually  present  an 
appearance  something  like  this :  On  the  front  side  of  the 
thunderstorm  there  are  grayish  white  or  reddish  clouds 
hanging  over  and  in  front  of  the  main  rain  cloud.  Above 
these,  dense  dark-gray  and  violet  cumulo-stratus  are  seen, 
as  also  dome-like  cumulus  clouds,  which  are  separated 
from  the  cumulo-stratus.  Often  these  are  interspersed  with 
one  or  more  thick  cumulo-stratus-like  cloud  layers,  and 
above  all  is  the  widely  distributed  cover  of  cirro-stratus. 

The  first  thunder  is  heard  before  the  cloud  reaches  the 
zenith,  and  the  first  rain  commences  after  it.  The  interval 
between  the  first  thunder  and  the  beginning  of  the  rain 
varies  from  a  few  minutes  to  perhaps  half  an  hour  or 
more.  Just  before  (usually  only  five  minutes  or  so)  the 
rain  begins,  there  comes  from  the  west  or  northwest  a 
brisk  wind,  which  suddenly  increases  in  violence,  and  be- 
comes a  squall,  which  is  the  name  given  to  a  sudden  vio- 
lent and  not  long-continued  wind ;  this,  however,  dies 
down  very  much,  after  the  rain  begins.  The  time  of 
heaviest  rainfall  varies:  sometimes  it  occurs  at  the  begin- 
ning, and  sometimes  in  the  latter  part,  of  the  time  when  the 
rain  falls.  The  thunder  is  loudest,  and  lightning  strokes 
occur  some  minutes  after  the  rain  begins. 


LOCAL  AND   MISCELLANEOUS   WINDS.  251 

Soon  after  this  the  western  horizon  loses  its  dark  aspect, 
and  begins  to  lighten  up  a  little,  and  finally  the  clouds 
break  there,  and  blue  sky  is  seen.  The  storm  clouds  then 
pass  by  overhead,  and  the  rain  ceases  some  minutes  before 
their  western  edge  reaches  the  zenith.  Although  the  tem- 
perature may  have  fallen  even  as  much  as  25°  F.  during 
the  storm,  yet  it  frequently  regains  its  former  condition  after 
the  storm  has  passed.  Rainbows  appear  in  the  latter  part 
of  the  rain  period  of  the  storm.  The  last  thunder  is  usu- 
ally heard  after  the  rear  edge  of  the  clouds  has  passed  the 
zenith.  Individual  thunderstorms  seldom  last  (between 
the  first  and  last  thunder)  over  two  hours,  but  frequently 
one  thunderstorm  follows  another  in  such  quick  succession 
as  to  appear  to  be  part  of  the  same  storm.  In  our  lati- 
tudes the  usual  direction  of  translation  of  thunderstorms 
is  easterly. 

Classes  of  Thunderstorms. -- Thunderstorms  are  now 
generally  divided  into  three  classes,  —  cyclonic,  heat,  and 
winter  thunderstorms. 

Cyclonic  thunderstorms  accompany  the  well-developed 
areas  of  low  barometric  pressure,  and  they  have  a  circula- 
tion somewhat  analogous  to  that  of  cyclones,  becoming  in 
extreme  cases  tornadoes. 

Heat  thunderstorms  are  the  result  of  the  local  heating 
of  the  lower  air,  which  makes  its  condition  unstable. 
They  need  for  their  development  a  moist  quiet  air  warmed 
by  the  sun's  rays,  and  they  occur  in  their  most  distinct 
state  outside  of  the  regions  of  strong  ascending  and  de- 
scending currents  of  cyclonic  and  anticyclonic  areas. 

Probably  many,  if  not  the  majority,  of  our  thunder- 
storms are  a  combination  of  these  two  forms. 

Winter  thunderstorms  occur  most  frequently  at  night, 
especially  in  high  latitudes,  and  they  are  more  frequent 


252 


ELEMENTARY  METEOROLOGY. 


'-^-IS  '££* 


near  the  coast  than  inland.     These  winter  storms  are  of 
more  limited  area,  travel  faster,  are  shorter  in  duration, 

and  their  lightning  is 
more  destructive,  than 
the  summer  thunder- 
storms. 

Heat  or  Stationary 
Thunderstorms.  —  In  a 
stationary  thunderstorm, 
that  is,  one  having  no  or 
but  very  slight  progres- 
sive motion,  we  have  at 
-5  first  an  ascending  cur- 

FIG.  78.— UPWARD    AIR    CURRENT    IN    THUNDER-     rent    of    moist    Warm    air 


over  some  limited  region. 


STORM  (AFTER  FERREL). 

Air  then  flows  in  from  all  sides,  and  a  vertical  circulation 
arises  (Fig.  78).  When  the  ascending  air  reaches  an  alti- 
tude where  its  dew-point  occurs,  then  condensation  begins, 
and  a  cloud  is  formed 
which  has  a  flat  base. 
Above  this,  precipita- 
tion occurs  ;  and  the  rain 
falling  through  the  air 
beneath  cools  it,  so  that 
it  contracts,  becoming 
heavier  than  the  sur- 
rounding air,  and,  being 
also  pressed  downward 
by  the  falling  rain,  falls 
downward,  after  over- 
coming the  gentle  upward  current  which  already  existed 
there.  The  movement  of  the  air  downward  causes  an  out- 
flow of  the  air  below  it,  and,  forcing  its  way  into  the  origi- 


a  c  c'  c 

FIG.  79.  —  AIR  CURRENTS  IN  THUNDERSTORM 
(AFTER  FERREL). 


LOCAL  AND   MISCELLANEOUS   WINDS.  253 

nai  ascending  current  which  surrounds  it,  causes  it  to  spread 
out,  thus  increasing  the  size  of  the  atmospheric  disturbance 
and  the  cloud  region  overhead.  The  air  thus  forced  out 
from  below  at  the  center  also  takes  part  in  the  vertical  cir- 
culation of  the  surrounding  ascending  air  (Fig.  79).  Since 
this  occurs  on  all  sides,  the  outer  upward  currents  are  ar- 
ranged symmetrically  around  the  inner  downward  current. 

There  is,  then,  a  central  region  of  greatest  air  pressure 
caused  by  this  central  cooling  and  downward  flow  of  air, 
which  reaches  its  maximum  at  the  earth's  surface;  and 
outward  from  this  the  temperature  increases  and  the  pres- 
sure decreases,  and  this  change  is  very  rapid  within  a 
short  horizontal  distance  at  the  dividing  line  between  the 
descending  and  ascending  currents.  It  is  in  this  region 
of  steep  gradients  that  the  squall  or  sudden  wind  occurs. 
The  air  which  descends  at  the  interior,  forces  its  way  out 
under  the  ascending  current  encircling  it,  and,  in  part  at 
least,  very  soon  joins  the  latter  in  its  upward  movement, 
after  its  horizontal  force  is  somewhat  spent.  By  this  mix- 
ture of  the  cool  interior  air  with  the  warmer  outer  air,  the 
latter  becomes  cool  on  the  inner  side,  and  changes  there  to 
a  descending  current,  so  that  the  ring  of  steep  gradient  and 
squalls  is  enlarged.  This  process  lasts  until  the  stable 
equilibrium  is  restored,  when  the  storm  disappears. 

Cyclonic  or  Progressive  Thunderstorms.  —  It  must  be 
rare,  at  least  in  middle  latitudes,  that  the  air  is  quiet  enough 
to  permit  the  formation  of  a  symmetrical  thunderstorm 
such  as  has  just  been  mentioned.  In  the  southern  and 
southeastern  parts  of  the  large  cyclones,  and  at  a  distance 
of  several  hundred  miles  from  the  center,  where  our  thun- 
derstorms usually  occur,  there  is  an  easterly  motion  (the 
wind  varying  from  northwest  to  southwest)  of  the  air.  It 
is  this  easterly  motion  which  carries  the  thunderstorm  on- 


254  •     ELEMENTARY   METEOROLOGY,, 

wards ;  and  since  the  motion  is  greater  above  than  below, 
new  masses  of  air  are  introduced  on  the  front  side  near 
the  earth's  surface,  and  thus  a  fresh  supply  of  moisture  is 
drawn  into  the  vertical  currents  to  keep  up  the  precipita- 
tion; and  on  the  rear  side  this  air  is  left  drier  and  cooler. 
Not  only  does  the  storm  progress  in  this  easterly  direction 
with  the  upper  air  currents,  but  it  increases  also  on  the 
side  of  greatest  moisture  and  heat,  which  in  this  case  is 
the  preceding  or  front  side. 

The  local  gradient  which  occurs  between  the  air  of  the 
inner  and  that  of  the  outer  regions,  and  which  is  indicated 
by  the  sudden  rise  of  barometric  pressure  at  the  central 
region  in  the  thunderstorm,  is  sufficient  to  cause  a  wind  as 
powerful  as  that  observed  at  the  front  edge  of  the  storm. 

Thunderstorms  seem  to  occur  under  much  the  same 
conditions  as  those  favorable  for  the  formation  of  torna- 
does. The  latter  are  nearly  always  accompanied  by  light- 
ning ;  and  where  the  tornado  is  too  far  distant  to  render 
its  distinctive  funnel  cloud  visible,  but  not  too  far  for  its 
thunder  to  be  heard  or  its  lightning  seen,  it  is  regarded  as 
a  thunderstorm. 

Progressive  Movement  of  Thunderstorms.  —  Although 
the  thunderstorm  thus  commences  small  in  size,  it  does 
not  remain  so.  As  the  storm  progresses  (moving  generally 
in  a  direction  somewhat  easterly),  it  broadens  out  so  that  it 
presents  an  ever-increasing  length  of  front,  until  it  is  dis- 
sipated or  else  breaks  up  into  smaller  storms.  This  is 
illustrated  in  the  following  diagram  (Fig.  80),  in  which  the 
relative  length  of  the  front  at  successive  hours  is  shown. 

Sometimes  thunderstorms  expand  in  all  directions  from 
a  central  point,  but  this  occurs  usually  when  the  storm  is 
very  local  and  stationary,  or  in  a  stationary  stage.  The 
direction  of  propagation  of  thunderstorms  is  usually  that  of 


LOCAL  AND   MISCELLANEOUS  WINDS.  255 

the  local  wind  belonging  to  the  cyclonic  system  in  which  it 
appears.  In  Europe,  thunderstorms  move  with  an  average 
velocity  of  from  22  to  25  miles  per  hour ;  and  in  general 
the  velocity  is  probably  nearly  equal  to  that  of  tornadoes. 

Time  of  Occurrence  of  Thunderstorms.  —  Our  thunder- 
storms occur  most  frequently  in  June,  July,  and  August,  and 
least  frequently  in  winter; 
they  occur  most  frequently 
in  the  middle  of  the  after- 
noon, and  least  frequently 
in  the  early  morning  hours. 
In  some  places  it  has  been 

FIG.  80.  —  PROGRESSIVE  GROWTH  OF  THUNDER- 

found   that   more  thunder-  STORMS. 

storms  occur  one  or  two  hours  after  midnight  than  in  the 

hours  just  preceding  and  following. 

Variation  of  Number  with  Latitude.  —  Thunderstorms 
are  in  general  most  frequent  near  the  equator,  and  decrease 
with  increase  of  latitude  ;  but  their  number  depends  also  on 
the  temperature,  the  moisture,  and  the  movement  of  the 
air.  For  instance,  in  the  tropics,  during  the  season  of 
calms,  thunderstorms  are  frequent ;  but  during  the  season 
of  trade  winds  they  are  rare,  because,  the  air  as  a  whole 
being  then  in  constant  motion,  the  initial  condition  of 
unstable  equilibrium  necessary  for  the  formation  of  thun- 
derstorms seldom  occurs. 

Changes  in  Meteorological  Elements  during  Thunder- 
storms. —  In  the  passage  of  thunderstorms  the  meteorolog- 
ical elements  undergo  the  following  changes :  Before  the 
thunderstorm  the  air  pressure  and  the  relative  humidity  de- 
crease, and  the  temperature  rises,  the  wind  being  generally 
weak ;  so  that  at  the  beginning  of  the  storm  the  pressure 
and  relative  humidity  are  at  their  lowest,  and  the  tempera- 
ture at  its  highest  point.  At  the  moment  of  the  bursting  of 


256 


ELEMENTARY   METEOROLOGY. 


the  storm  the  air  pressure  and  relative  humidity  increase 
very  rapidly,  and  the  temperature  falls ;  the  wind  also 
suddenly  becomes  strong,  and  sometimes  it  as  suddenly 
subsides  almost  immediately  afterwards,  while  at  other 
times  it  increases  until  near  the  close  of  the  thunderstorm. 
Towards  or  at  the  end  of  the  thunderstorm,  the  air  pressure 


28°C. 
26°  4 
24°  3 
22°  2 
20°  1 
18°  0 

81 
53 
26 
0 

N.W. 
W 


s.w. 

s 

754  S.E, 
753 

"752  mm, 


'Bctr'on 


FIG.  81.  —  METEOROLOGICAL  ELEMENTS  D.URING  A  THUNDERSTORM  (SPRUNG). 

and  relative  humidity  reach  their  maximum,  and  the  tem- 
perature its  minimum.  These  relations  are  shown  by  the 
preceding  diagram  (Fig.  81),  which  shows  the  variation  of 
the  meteorological  elements,  at  a  place  in  the  path  of  a 
thunderstorm,  during  the  afternoon  and  evening  of  the  day 
on  which  a  thunderstorm  occurred  at  half-past  six  o'clock. 


LOCAL  AND  MISCELLANEOUS  WINDS.       257 

The  rain  areas  in  progressive  thunderstorms  increase 
with  the  spreading-out  of  the  upward  air  currents  and 
gradual  growth  of  the  storm; 
and  at  any  one  time  the  rain 
area  has  an  average  form  as 
shown  in  Fig.  82.  The  thun- 
derstorm is  carried  along  by  the 
wind  which  is  least  at  the  sur- 
face of  the  earth;  so  that  the 
warm  moist  air  on  the  front 
side  feeds  the  air  in  the  thun- 
derstorm  like  a  lower  current  FIG.  82. —FORM  OF  RAW  AREA  IN  A 
in  a  westerly  direction,  and  the 

storm  grows  in  extent  towards  this  lower  feeding  cur- 
rent, which  supplies  it  with  moisture.  In  the  rear  the 
storm  dies  out  from  lack  of  moisture  to  sustain  it ;  also  the 
storm  spreads  toward  the  south  when  warm  moist  southern 
winds  blow  into  it,  and  with  the  diminution  of  this  moisture 
by  precipitation  there  is  a  dying-out  of  the  storm.  The 
growth  of  the  storm  is  therefore  toward  the  east  and 
south.  On  the  north  side  the  storm  increases  in  size  but 
slowly  and  in  a  more  regular  form,  and  is  even  wider 
than  on  the  south  side,  because  on  the  north  the  changes 
of  moisture  from  one  stage  to  another  take  place  less 
rapidly,  and  the  moisture  is  held  in  the  air  longer. 

Ideal  Sketch  of  a  Thunderstorm.  —  The  following  dia- 
gram (Fig.  83)  gives  a  detailed  sketch,  in  vertical  section, 
of  the  clouds  and  their  circulation  in  a  thunderstorm,  or  in 
a  rain  squall  not  accompanied  by  thunder. 

On  the  right-hand  edge  or  front  there  is  seen  the  outflow  of  air 
below,  with  a  gentler  inflow  towards  the  cloud  above.  In  the  rainy 
region,  shown  by  the  shading,  there  is  a  downward  flow  of  air,  and 
to  the  rear  of  this  is  the  upward  current  already  mentioned.  The 


258 


ELEMENTARY   METEOROLOGY. 


lower  edge  of  the  roll-like  clouds  at  b  and  (£)  may  be  from  1,000  or 
1,200  to  1,500  feet  above  the  ground,  while  the  dome-like  vault  of  the 
cloud  in  the  rain  section  is  perhaps  2,500  feet.  At  (J)  there  may  be 
almost  or  quite  a  cessation  of  the  rain,  and  then  it  may  commence 
again,  and  perhaps  another  cessation  may  occur  as  shown.  The  cloud 


FIG.  83. -CLOUDS  AND  WINDS  IN  A  THUNDER  SQUALL  WHICH  is  MOVING  TOWARDS  THE 
RIGHT  (VERTICAL  SECTION).     (AFTER  KOPPEN.) 

forms  are  distinguished  as  follows :  At  c  are  the  cumulus  cloud  heads 
or  tops  projecting  from  the  mass  of  dark  storm  cloud  a.  At  b  and  (6) 
are  dense  rolls  of  clouds  hanging  down  lower  than  the  main  mass.  At 
ee  there  is  a  continuous  cirro-stratus  layer ;  and  lower,  at  e',  is  another 


FIG.  84.  —  CLOUDS  AND  WINDS  IN  A  HAIL  SQUALL. 

layer  partly  broken,  and  pierced  by  the  heads  of  the  cumulus  clouds 
below.  At  d  instead  of  rounded  tops  there  are  cloud  tops  of  fantastic 
forms  resulting  from  the  outstreaming  of  the  air.  The  downward  cur- 
rent between  b  and  (b)  is  due  to  the  heaping-up  of  air  at  a,  and  replaces 


LOCAL  AND   MISCELLANEOUS   WINDS.  259 

the  air  carried  upward  on  either  side  at  b  and  (£).  At  the  rear  of  the 
storm  are  frequently  seen,  as  at  g,  isolated  cloudlets  due  to  the  local 
complete  saturation  of  the  cold  air  just  after  the  storm  has  passed. 

In  case  of  hail  formation  and  hail  fall,  the  frozen  rain  will  fall  in  the 
region  at  the  front  side  shown  by  the  coarser  dotted  shading  in  Fig.  84. 

Hailstones  may  form  in  the  upward  current  of  moist  air,  and  then 
drop  out  to  one  side  of  the  upward  current  down  into  the  region  of  rain, 
where  they  will  have  water  added  to  their  exteriors  ;  and  then  they  may 
be  carried  up,  in  the  ascending  current  into  which  they  have  fallen,  into 
the  region  of  freezing.  The  hailstones  may  become  largely  increased 
in  size  by  repetition  of  this  process.  Sometimes  the  central  kernel  of 
a  hailstone  is  a  snowflake,  which  at  first  becomes  thoroughly  wet  by  con- 
tact with  rain,  and  then  is  carried  up  by  an  air  current  to  the  region  of 
freezing  ;  and  then,  following  the  process  just  described  for  a  hailstone,  it 
may  increase  to  a  very  large  size.  A  variety  of  such  processes  must  occur 
in  order  to  account  for  the  different  kinds  of  hailstones  which  fall  during 
storms  of  this  kind. 

Thunderstorms,  tornadoes,  and  hailstorms  have  a  common  relation  to 
cyclones  as  far  as  position  is  concerned,  since  they  all  depend  on  the 
same  condition  of  great  humidity  and  instability  of  the  atmosphere.  In 
addition  to  those  conditions  necessary  for  the  formation  of  a  thunder 
squall,  tornadoes  require  the  local  conditions  which  cause  the  gyratory 
motions  peculiar  to  them ;  and  hailstorms  require  the  powerful  ascend- 
ing current  which  extends  up  to  altitudes  where  freezing  can  take  place. 


SPOUTS. 

Spouts  are  cloud-like  or  fog-like  phenomena  of  a  slender, 
more  or  less  tapering  form,  like  a  long  trunk  or  funnel, 
formed  in  the  air  under  certain  atmospheric  conditions 
The  funnel-shaped  cloud  of  a  tornado  is  a  spout,  but  there 
are  also  column-like  spouts  which  occux-  in  fair  weather, 
especially  over  water  surfaces,  and  which  extend  from  the 
earth's  surface  to  the  clouds.  These  are  waterspouts,  and 
are  the  cloud  brought  down  to  the  earth  by  the  rapid  gyra- 
tory motion  such  as  occurs  at  the  center  of  a  tornado. 


260 


ELEMENTARY   METEOROLOGY. 


Spouts  are  due  to  the  rarefaction  of  the  air  caused  by 
the  centrifugal  force  driving  some  of  the  air  from  the 
center  of  a  rotating  column  of  air  in  which  the  motion  is 
spirally  inward  and  upward.  As  the  moisture-laden  air 
enters  this  central  region,  where  the  air  pressure  is  reduced 

so  much  as  to  bring  the 
air  temperature  down  to 
the  dew-point,  the  vapor 
is  condensed  into  cloud. 
It  is  this  cloud  of  con- 
densed moisture  which 
we  see  as  a  dark  column. 
Water  is  sucked  up  some 
distance  above  the  water 
surface  at  the  foot  of  the 
column,  but  probably  not 
very  high  up  into  the 
air.  The  gyrations  being 
most  violent  at  certain  al- 
titudes above  the  earth's 
surface,  the  central  rare- 
faction is  in  the  begin- 
ning greatest  up  there ; 
and  the  funnel  or  spout 
cloud  forms  there  first, 
and  afterwards  extends 

down  to  the  earth's  surface  if  the  gyratory  velocities 
are  sufficiently  great  to  diminish  the  air  pressure  in  the 
central  region  by  such  an  amount  as  to  cause  conden- 
sation of  the  moisture  in  the  lowest  air  layer.  Fre- 
quently spouts  of  little  energy  are  not  visible  clear  to  the 
earth's  surface.  Fig.  85  illustrates  waterspout  phenomena 
of  various  degrees  of  energy,  and  shows  how  a  water 


FIG.  85.  —  WATERSPOUTS. 


LOCAL  AND  MISCELLANEOUS  WINDS.       261 

surface  is  stirred  up  by  the  whirlwind  at  the  base  of  the 
spout 

Dust  Whirlwinds  are  really  spouts  of  feeble  energy,  in 
which  the  air  is  too  dry  to  permit  the  cloud  formation  by 
the  condensation  of  vapor,  The  dust,  however,  is  carried 
inwards  and  upwards,  and  is  held  suspended  in  the  air 
where  the  motions  are  strong  enough  to-  sustain  it;  and 
the  outline  of  this  region  is  rendered  visible  by  the  dust 
particles. 

White  Squalls,  or. fair-weather  whirlwinds,  occur  when 
the  conditions  are  ripe  for  spout  formation,  but  there  is 
not  enough  moisture  present  to  form  the  cloud  through- 
out the  length  of  the  central  vortex.  Up  at  a  considerable 
altitude,  however,  a  cloud  may  be  formed  which  will  indi- 
cate the  location  of  the  top  of  the  vortex;  and  at  the  bot- 
tom the  gyratory  inflowing  wind  may  create  a  disturbance 
which  is  especially  noticeable  over  a  water  surface,  and 
in  well-developed  cases  proves  of  great  danger  to  ships. 
These  squalls  are  sometimes  called  bull's-eye  squalls,  on 
account  of  the  peculiar  appearance  of  the  small  isolated 
cloud  which  marks  the  top  of  the  invisible  spout  at  the 
center  of  the  whirlwind. 

Cloud-bursts  are  sudden  and  excessive  downpours  of 
rain,  or  rain  and  hail,  which  have  been  carried  upward  or 
merely  sustained  and  kept  from  falling  by  the  ascending 
air  currents,  until  a  large  amount  has  been  accumulated 
aloft,  when,  by  some  weakening  or  breaking-up  of  the 
ascending  currents,  the  whole  or  a  part  of  the  accumu- 
lation suddenly  falls  to  the  ground.  Cloud-bursts  are  of 
most  frequent  occurrence  in  connection  with  tornadoes, 
where  the  immense  velocity  of  the  ascending  current  is 
favorable  to  the  collection  and  support  of  great  masses  of 
water. 


262  ELEMENTARY   METEOROLOGY. 

PERIODIC  LOCAL  WINDS. 

Periodic  Local  Winds  are  periodic  winds  which  do  not 
depend  on  the  general  or  secondary  circulation  of  the 
atmosphere.  They  are  the  land  and  sea  breezes,  moun- 
tain and  valley  winds,  and  monsoons. 

Land  and  Sea  Breezes  occur  daily  on  the  coasts  of  large 
bodies  of  water.  When  the  sun  rises,  the  land  is  warmed 
more  rapidly  than  the  water  surface ;  and  this  heat,  being 
communicated  to  the  air  above  the  land,  causes  an  expan- 
sion upwards,  and  the  surfaces  of  equal  air  pressure  are 
thus  raised  higher  over  the  land  than  over  the  sea.  The 
air  over  the  land  flows  down  these  inclined  surfaces  (so  to 
speak)  towards  the  sea,  and  the  pressure  of  the  air  over 
the  sea  is  thus  increased,  and  the  air  pressure  over  the  land 
correspondingly  decreased.  The  increased  pressure  over 
the  sea  causes  a  return  surface  wind  to  set  in  toward  the 
land  region  of  deficient  pressure.  This  latter  air  current 
commences  at  some  distance  out  at  sea,  away  from  the 
shore,  and  moves  inward  towards  the  land.  This  circula- 
tion is  kept  up  until  the  air  above  the  land  is  no  longer 
warmer  than  that  over  the  water. 

At  evening,  when  the  sun  goes  down,  the  land  cools 
more  rapidly  than  the  water,  and  the  air  over  the  land 
becomes  cooled  more  rapidly  than  that  over  the  sea.  The 
result  is,  that  the  surfaces  of  equal  air  pressure  are  lowered 
over  the  land,  and  the  air  above  flows  from  above  the  sea 
down  these  surfaces,  causing  an  excess  of  pressure  over  the 
land,  and  an  under  wind  from  the  land  to  the  region  of  defi- 
cient pressure  over  the  sea.  This  land  breeze  continues 
until  the  temperature  and  pressure  differences  are  adjusted. 

Mountain  and  Valley  Winds  occur  in  mountainous 
regions  in  relatively  quiet  conditions  of  the  air.  There  is 


LOCAL  AND   MISCELLANEOUS   WINDS.  263 

then,  especially  in  clear  weather,  an  upward  motion  from  the 
valleys  during  the  day,  and  a  downward  motion  towards 
the  valleys  at  night.  These  winds  have  some  analogy  to 
the  land  and  sea  breezes.  In  the  night  the  cold  denser  air 
descends  from  the  mountain  tops,  which  cool  very  rapidly 
by  radiation,  toward  the  valleys  below.  These  downward 
currents  commence  first  in  the  narrow  valleys  on  the 
mountain  side,  in  which  the  air  is  least  heated  during  the 
day ;  but  the  descent  soon  becomes  general,  and  the  air 
flows  down  to  wider  valleys  below.  In  the  morning  the 
air  over  lower  levels  of  the  ground  becomes  heated,  and 
the  surfaces  of  equal  air  pressure  over  those  levels  at  some 
little  distance  away  from  the  mountain  are  elevated,  but 
directly  on  the  mountain  they  are  not,  because  there  is  no 
air  below  to  expand  upward;  so  that  the  air  flows  down 
these  nearly  horizontal  surfaces  towards  the  upper  parts  of 
the  mountain,  which  it  strikes  at  such  an  angle  as  to  de- 
flect it  upward  along  the  mountain  side.  In  addition  to  this 
motion,  there  is  an  upward  motion  along  the  mountain  side, 
due  to  the  action  of  the  sun's  rays  in  the  warming  of  the 
ground  along  the  slope  and  the  air  immediately  above  it ; 
so  that  the  upward  motion  of  the  air  along  the  surface  of 
the  ground  is  the  result  of  these  two  motions  combined. 
The  accumulation  of  this  air  at  the  top  of  the  mountain  is 
prevented  by  horizontal  currents  which  exist  there. 

MISCELLANEOUS  WINDS. 

Under  this  title  we  shall  mention  certain  winds  which 
have  distinctive  features,  and  have  therefore  received 
separate  names.  Some  of  them  belong  to  the  great  pri- 
mary circulation  of  the  atmosphere,  and  some  to  the  second- 
ary circulation,  and  others  are  of  local  origin. 

WALDO    METEOR.  —  1 6 


264  ELEMENTARY   METEOROLOGY, 

Cold  and  Hot  Winds.  —  When  the  cyclonic  and  anti- 
cyclonic  conditions  of  the  middle  latitudes  are  such  as  to 
carry  to  warmer  regions  the  air  which  has  become  exces- 
sively cooled  by  local  causes,  there  arise  the  cold  winds 
which  have  received  their  distinctive  names  in  the  various 
regions  in  which  they  occur.  When  the  conditions  are 
such  as  to  carry  to  colder  regions  air  which  has  been  made 
excessively  warm  by  local  causes,  warm  winds  are  experi- 
enced. The  warm  winds  usually  have  a  poleward  and  the 
cold  winds  an  equatorward  movement,  but  either  wind 
may  have  a  motion  in  other  directions. 

Cold  Winds,  —  The  cold  north  and  northwest  winds  of 
the  United  States,  called  in  the  south  northers  and  in  the 
north  blizzards,  the  bora  of  the  Adriatic,  the  mistral  of 
Mediterranean  France,  the  buran  of  Russia,  and  the  pam- 
pero of  South  America  from  the  southwest,1  are  due  to  cur- 
rents of  cold,  dry,  dense  air  which  flow  from  the  cooler  to 
the  warmer  latitudes  under  the  control  of  the  secondary 
atmospheric  disturbances.  These  cold  winds  occur  usually 
when  the  temperature  changes  are  very  rapid  within  short 
geographical  distances,  and  consequently  the  cold  air  need 
not  be  blown  far  to  reach  regions  of  greater  warmth.  They 
consequently  occur  in  winter,  and  most  frequently  in  those 
regions  where  the  temperature  gradients  are  steepest. 
The  extent  to  which  they  penetrate  the  warmer  regions 
depends  on  their  persistency  and  the  velocity  as  well  as 
the  volume  of  the  moving  mass  of  cold  air. 

When  the  region  from  which  these  northerly  (in  the 
northern  hemisphere)  winds  blow  is  very  much  elevated, 
as  for  instance  is  very  markedly  the  case  along  the  north- 
ern shores  of  the  Adriatic  Sea,  then  the  air  which  has  be- 

1  It  must  be  remembered  that  in  the  southern  hemisphere  the  polar  winds 
come  from  the  south. 


LOCAL  AND   MISCELLANEOUS   WINDS.  265 

come  excessively  cooled  by  radiation  on  these  highlands  is 
partly  blown,  and  partly  descends  from  its  own  greater  den- 
sity, on  to  the  low  lands  or  sea  below;  and,  notwithstanding 
the  adiabatic  warming,  it  reaches  the  low  levels  still  much 
cooler  than  the  surrounding  air,  which  it  would  not  do  un- 
less there  were  steep  horizontal  temperature  gradients  in 
the  air.  Thus  are  formed  the  bora  of  the  Adriatic,  and 
the  tramontane*  negra  or  black  norther  of  Greece. 

Hot  Winds.  —  The  hot  winds  from  a  southerly  direction, 
—  the  sirocco  of  the  central  and  western  Mediterranean 
Sea,  the  leste  of  Madeira,  the  kahnisin  of  Egypt,  the  leveche 
of  Spain,  or  our  own  hot  winds  of  the  central  and  eastern 
United  States,  —  and  the  zondas  of  the  South  American 
pampas  from  a  northerly  direction,1  are  likewise  due  to  the 
influence  of  the  secondary  atmospheric  disturbances  of 
middle  latitudes,  and  are  the  reverse  of  the  cold  winds 
above  described.  The  air  is  blown  from  the  excessively 
heated  regions  to  the  cooler.  Sometimes  it  is  dry,  as  in 
the  case  of  the  dry  sirocco  of  Spain  and  occasionally  of 
Italy  and  Sicily,  and  the  hot  winds  of  our  great  plains ; 
and  sometimes  it  is  very  moist,  as  in  the  case  of  the  usual 
Italian  or  Sicilian  sirocco.  In  the  latter  case  the  very  dry 
hot  air  is  blown  from  the  Sahara  and  the  northern  coast 
of  Africa  across  the  Mediterranean  Sea,  where  it  takes  up 
the  moisture  to  almost  the  point  of  saturation ;  and  then  it 
reaches  the  northern  shores  of  the  sea  very  warm  and 
very  moist,  which  renders  the  air  very  oppressive. 

Foehn  Winds.  —  The  foehn  winds,  so  called  by  the 
natives  in  the  Alps,  where  they  frequently  occur,  are  warm 
dry  winds  or  hot  waves  peculiar  to  some  mountainous  re- 
gions, and  are  the  result  of  the  following  process :  The 

1  In  the  southern  hemisphere  the  equatorial  winds  come  from  the  north. 


266  ELEMENTARY   METEOROLOGY. 

warm  moist  air  is  blown  against  the  side  of  a  mountain 
range,  and  a  not  necessarily  rapid  upward  air  current  arises 
along  the  slope ;  and  condensation  usually  takes  place  either 
by  the  cooling  of  the  air  coming  in  contact  with  the  colder 
mountain  top  or  by  adiabatic  cooling  of  the  ascending  air, 
or  by  both.  When  the  moisture  has  thus  been  abstracted 
from  the  air  in  its  passage  over  the  mountains,  the  air, 
which  begins  its  descent  on  the  other  side  of  the  mountain 
range,  is  much  drier;  and  in  this  condition  its  adiabatic 
increase  in  temperature  as  it  moves  downward  is  much 
more  rapid  than  the  adiabatic  cooling  in  its  ascent,  for  the 
latter  was  retarded  by  the  liberated  latent  heat  when  the 
moisture  of  the  air  was  condensed ;  and  with  the  descent 
the  relative  humidity  decreases.  Thus,  when  it  reaches 
low  altitudes,  the  air  becomes  very  warm  and  very  dry. 

The  decrease  in  temperature  for  saturated  ascending  air  is  only  about 
half  as  much  per  100  feet  as  the  increase  of  temperature  for  the  dry 
descending  air  which  is  subject  to  dynamic  heating  by  compression. 
The  greater  the  amount  of  moisture  lost  out  of  the  air,  and  the  farther 
the  descent  of  the  air,  the  hotter  it  becomes,  and  the  more  intense  are 
the  foehn  characteristics. 

The  degree  of  intensity  of  heat  in  the  foehn  depends 
on  the  amount  of  water  lost  by  condensation  high  up  in 
the  mountains,  and  on  the  distance  of  the  descent  of  the 
air.  If  no  water  were  lost  out  of  the  air,  there  would  be 
no  foe/in. 

One  peculiarity  of  these  foehn  winds  is  that  the  descent  of  air  fre- 
quently, or  perhaps  generally,  occurs  in  isolated  or  disconnected  streams, 
and  thus  the  resulting  excessively  heated  condition  of  the  air  below  is  not 
continuous,  and  is  found  only  at  intervals  ;  the  intervening  air  not  being 
much,  if  any,  above  a  natural  temperature.  Winds  of  a.  foe/in-like  char- 
acter are  found  to  the  leeward  in  most  mountain  regions  where  warm 
moist  winds  blow  against  mountain  ranges.  The  hot  Chinook  winds  of 


LOCAL  AND  MISCELLANEOUS  WINDS.        267 

the  western  part  of  America  at  the  eastern  slope  of  the  Rocky  Moun- 
tains, and  on  the  plains  at  their  base,  are  due  to  this  foe/m  process,  as 
this  drying  and  warming  of  moist  winds  is  called.  They  are  especially 
frequent  near  the  northern  border  of  the  United  States,  where  the  ex- 
cessively moist  air  from  the  Pacific  Ocean  coast  of  Oregon  and  Wash- 
ington is  carried  inland,  and  it  is  probable  that  they  extend  southward 
even  to  Texas. 

A  single  case,  which  occurred  at  Fort  Assiniboine,  Montana,  on  Jan. 
19,  1892,  may  be  mentioned,  in  which  the  temperature  rose  about  43°  F« 
between  2  A.M.  and  2.15  A.M.,  changing  from  —5.5°  F.  to  37.5°  F. 
within  these  15  minutes.  In  some  cases  the  temperature  rises  80°  F. 
in  the  course  of  6  or  8  hours. 

It  is  very  probable  that  the  foehn  process  takes  place 
in  the  free  atmosphere,  where  air  currents  ascend,  and 
some  water  is  lost  by  rainfall ;  and  then  the  same  air 
enters  a  descending  current,  and  is  brought  down  to  or 
near  the  ground  again.  This  would  naturally  occur  in 
the  circulation  between  cyclones  and  anticyclones,  where 
the  air  ascends  in  the  former,  and,  after  losing  some  of  its 
moisture,  descends  in  the  latter,  and  reaches  its  former  al- 
titude warmer  than  when  it  started.  The  same  reasoning 
also  applies  to  the  case  of  the  rising  and  falling  air  in  the 
local  thunderstorm  and  tornado  phenomena. 

The  effect  of  the  hot,  dry  foehn  winds  is  very  marked. 
When  they  occur  in  the  cold  season  in  regions  where  snow 
falls,  the  snow  disappears  almost  as  if  by  magic,  and  these 
warm  winds  are  thus  sometimes  called  snow  eaters.  When 
they  occur  in  the  warm  season,  vegetation  is  frequently 
withered,  and  growing  crops  entirely  destroyed. 

Avalanche  Winds  are  the  movements  of  air  masses  which 
are  pushed  ahead  of  masses  of  land  or  snow  as  they  de- 
scend mountain  sides  in  land  or  snow  slides.  The  mass  of 
air  is  compressed  and  becomes  locally  denser-as  it  is  pushed 
ahead  of  the  moving  substance.  This  air  movement  is 


268  ELEMENTARY   METEOROLOGY. 

of  such  violence  that  it  does  great  damage  even  at  a 
distance  of  many  feet  from  the  solid  mass  of  snow  or 
earth  moved  in  the  avalanche.  In  the  case  of  a  snow 
avalanche,  the  limits  of  these  winds  are  well  shown  by  the 
cloud  of  snow  particles  which  they  carry  forward.  The 
effects  of  such  winds  have  been  especially  noticeable  in 
Switzerland,  where  trees  have  been  broken  off  by  the  wind 
at  a  distance  of  1,500  feet  from  the  mass  of  snow,  and  the 
particles  of  snow  driven  to  a  distance  of  more  than  a  mile 
from  it 


CHAPTER   XI. 
WEATHER  AND  WEATHER  PREDICTIONS 

Main  Features.  —  By  the  term  iveather  we  mean  the 
atmospheric  conditions  as  shown  by  the  meteorological 
elements  at  a  particular  time  or  during  a  short  specified 
period.  Climate  is  the  aggregate  of  weather  conditions, 
and  weather  is  but  a  phase  of  climate.  Thus  we  might 
speak  of  the  weather  at  any  instant  or  for  a  day,  season, 
or  even  a  year,  but  not  for  such  a  long  period  of  years  as 
would  give  the  average  conditions.  These  last  with  their 
oscillations  would  be  climate.  The  degree  of  heat,  the 
amount  of  moisture,  precipitation,  and  cloud,  and  the  di- 
rection and  force  of  the  winds,  are  the  main  features  of 
our  weather.  There  are  certain  combinations  of  conditions 
which  occur  together.  Thus,  in  general,  heat  and  wetness, 
and  cold  and  dryness,  go  hand  in  hand  ;  although  in  middle 
and  polar  latitudes,  where  the  seasonal  changes  are  most 
marked,  in  the  summer  time  cold  and  wetness,  and  in  the 
winter  time  cold  and  dryness,  go  together. 

Absolute  and  Relative  Weather  Conditions.  —  In  the 
different  localities  on  the  earth's  surface  we  get  used  to 
certain  normal  or  average  conditions  of  weather,  and  the 
ordinary  or  average  diurnal  changes  where  such  exist ;  but 
any  accidental  deviation  from  these  average  conditions  and 
changes  attracts  our  notice. 

Thus,  when  we  speak  of  a  hot  day,  it  usually  means  hot 
in  relation  to  the  average  temperature  at  that  hour  and 

269 


2/O  ELEMENTARY   METEOROLOGY. 

season  of  the  year.  A  condition  which  we  should  term 
hot  at  one  hour  of  the  day  might  be  cool  when  measured 
by  that  usual  at  another  hour.  Thus,  in  summer  a  temper- 
ature of  80°  F.  at  eight  o'clock  in  the  morning  would  seem 
hot ;  but  the  same  temperature  at  two  o'clock  in  the  after- 
noon would  not  seem  hot.  The  expressions  which  we 
apply  to  heat  and  cold  are  relative  in  their  significance, 
and  we  make  allowance  for  the  diurnal  changes. 

This  is  not  so  much  the  case  when  we  speak  of  the  wet 
or  dry  weather,  for  then  we  have  the  absolute  and  distinct 
dividing  line  between  precipitation  and  no  precipitation. 
The  dampness  of  the  air  without  precipitation  is  esti- 
mated on  a  scale  reaching  from  extreme  dryness  with  a 
clear  sky  to  the  dense  cloudiness  of  fog. 

The  wind  ranging  from  a  calm  to  a  storm  is  a  prominent 
feature  of  the  weather ;  and  it  is  partly  noticeable  in  its 
direct  effects,  as  when  it  moves  branches  of  trees,  and 
partly  in  its  indirect  effects,  as  when  it  causes  rapid  evapo- 
ration or  blows  the  cold  air  through  our  clothing,  and 
makes  the  cold  more  readily  felt. 

In  going  from  a  region  with  one  climate  to  that  with 
another,  or,  as  we  say,  from  one  climate  to  another,  one 
must  become  accustomed  to  the  average  conditions  in  the 
new  climate  before  he  can  judge  of  the  weather  and  its 
changes  by  the  same  standard  as  that  customarily  applied 
in  that  region. 

Seasonal  Weather.  —  During  long  periods  of  time  which 
take  account  of  average  conditions  only,  the  weather  con- 
ditions are  seasonal ;  and  although  there  may  occur  during 
any  one  season  marked  departures  from  the  averages  of 
many  seasons,  yet  these  irregularities  are  small  as  com- 
pared with  those  which  occur  during  short  intervals  of 
time. 


WEATHER  AND   WEATHER   PREDICTIONS.  2/1 

If  during  some  particular  season  the  average  temperature  is,  say, 
only  5°  F.  higher,  or  the  rainfall  a  few  inches  greater,  than  usual,  then 
we  speak  of  it  as  being  an  exceptionally  warm  or  wet  season.  The 
study  of  these  long-period  weather  conditions  belongs  more  properly  to 
climatology. 

Current  Weather  Conditions  are  studied  in  a  very  different 
manner  from  the  average  conditions  just  mentioned.  The 
true  cause  of  the  peculiar  weather  conditions  at  a  particu- 
lar time  at  some  one  place,  and  the  relation  of  these  con- 
ditions to  those  existing  at  the  same  time  at  adjacent  or 
even  quite  distant  places,  have  been  found  out  by  writing 
on  maps  of  extensive  regions  the  meteorological  conditions 
existing  simultaneously  at  a  great  number  of  places  in- 
cluded on  the  maps.  Such  maps  are  called  weather  maps ; 
and  they  not  only  permit  us  to  see  what  kind  of  weather 
actually  existed  at  the  time  the  observations  were  made, 
but,  as  we  shall  see,  they  also  enable  us  to  say  with  a 
considerable  degree  of  certainty  what  the  weather  has 
been  for  a  day  or  two  before,  and  what  it  will  be  for  a 
day  or  two  after,  that  time ;  and  this  last  we  call  weather 
predicting. 

In  the  present  chapter  we  shall  mainly  consider  these 
weather  maps  and  the  methods  of  using  them  for  predict- 
ing the  weather,  or  for  weather  forecasting ;  and  we  shall 
confine  ourselves  to  the  consideration  of  the  middle  lati- 
tudes of  the  northern  hemisphere,  as  these  are  the  regions 
in  which  we  are  most  interested,  and  where  such  studies 
are  mainly  carried  on. 

It  has  been  found  that  our  weather  in  these  latitudes 
is  mainly  conditioned  by  the  phenomena  attending  the 
passage  of  the  cyclones  and  anticyclones  moving  in  the 
general  direction  from  west  towards  the  east,  or  sometimes 
from  the  south  towards  the  northeast,  as  already  men- 


(272) 


WEATHER  AND    WEATHER   PREDICTIONS.  2/3 

tioned.  These  phenomena,  and  their  relation  to  cyclonic 
and  anticyclonic  areas,  have  already  been  described  in 
preceding  chapters,  where  the  distribution  of  the  meteor- 
ological elements  in  cyclones  and  anticyclones  is  given. 

Weather  Conditions  in  a  Cyclone  and  an  Anticyclone.  — 
Fig.  86  shows  the  combined  weather  conditions  of  a  cy- 
clonic and  an  anticyclonic  area.  On  the  front  (eastern) 
side  of  the  cyclone  we  see  at  first  high  cirrus  clouds  moving 
toward  the  east  or  northeast,  and  towards  the  center  lower 
clouds  moving  from  the  east  or  southeast,  with  increased 
cloudiness  and  rain ;  the  air  pressure  diminishes,  but  the 
temperature  does  not  change  much.  Near  the  center  of 
the  cyclone  the  rain  ceases,  and  cloudiness  diminishes. 

On  the  rear  (western)  side  of  the  center  there  is  some 
cloudiness  and  perhaps  showers,  and  proceeding  from  the 
center  the  temperature  falls,  the  air  pressure  increases,  and 
the  wind  blows  from  the  west  and  northwest. 

At  the  south  side  of  the  cyclone,  proceeding  from  the 
front  (east)  to  the  rear  (west),  the  winds  veer  around  from 
southeast  through  south  to  the  west,  and  the  weather 
changes  from  rainy  to  clearing,  or  showery  before  finally 
clearing  with  approach  to  the  anticyclonic  area.  The  tem- 
perature is  at  first  quite  warm  and  constant,  but  with  the 
shifting  of  the  winds  to  west  and  northwest,  it  rapidly 
becomes  colder. 

At  the  north  side  of  the  cyclone,  proceeding  from  the 
front  (east)  to  the  rear  (west),  the  wind  changes  from  east 
through  north  to  northwest.  With  the  east  and  northeast 
winds  there  are  clouds  and  rain,  but  clear  weather  with 
north  and  northwest  winds.  The  temperature  falls  rapidly 
with  the  beginning  of  the  north  wind. 

For  the  anticyclonic  area  there  is  usually  clear,  cool 
weather,  with  north  and  northwest  winds  on  the  front  side, 


2/4  ELEMENTARY   METEOROLOGY. 

and  east  winds  on  the  rear.  On  the  south  (passing  from 
east  to  west),  the  winds  shift  from  northeast  through 
east  to  southeast,  the  temperature  at  first  falling,  and  then 
rising ;  while  on  the  north  they  shift  from  northwest 
through  west  to  southern,  the  temperature  not  varying 
much.  On  the  west  side  (passing  from  north  to  south), 
the  winds  shift  from  southeast  to  northeast  through  the 
east,  the  temperature  grows  warmer,  and  the  clouds  of 
the  following  cyclone  begin  to  make  their  appearance. 

Methods  of  Making  Weather  Predictions.  --  Weather 
predictions  are  made,  first,  from  local  observations,  and 
refer  to  the  place  at  which  they  are  made  ;  second,  from 
weather  charts  of  an  extended  region,  and  refer  to  any 
region  on  the  chart ;  third,  from  weather  charts  and  local 
observations,  and  refer  to  the  region  at  and  around  the 
place  at  which  the  local  observations  are  made. 

Weather  Predictions  from  Local  Observations.  —  It  has 
been  found  that  the  probability  of  the  continuation  of 
existing  weather  is  greater  than  that  for  a  change  of 
weather ;  and  so  we  may  assume  that  the  present  state  will 
continue,  unless  some  phenomenon  presents  itself  which 
foretells  a  change  in  the  weather.  We  have  such  phe- 
nomena in  the  movement  and  condition  of  the  atmosphere, 
as  shown  by  the  direction  of  the  wind,  and  kinds  of  the 
clouds.  If  we  take  as  a  standard  or  normal  condition  dry 
weather  and  a  clear  or  but  slightly  cloudy  sky,  then  the 
change  to  generally  wet  weather  and  an  overcast  sky  is 
heralded  by  the  appearance  of  the  cirrus,  cirro-cumulus, 
cirro-stratus,  and  stratus  clouds  which  almost  invariably 
precede,  or  we  may  say  outrun,  a  cyclonic  area. 

The  next  point  is  to  locate  with  some  accuracy  the 
center  of  the  approaching  storm.  This  can  be  done 
by  placing  the  back  towards  the  direction  of  approach 


WEATHER  AND   WEATHER   PREDICTIONS.  2/5 

of  the  surface  wind,  when  the  region  of  low  air  pressure 
(cyclonic  area)  will  be  on  the  left  and  a  little  in  advance, 
and  that  of  higher  pressure  (anticyclone)  on  the  right 
and  a  little  in  the  rear,  of  the  position  occupied  by  the 
observer.  The  latter  can  then  make  a  guess  as  to  which 
of  the  quadrants  of  the  storm  area  will  pass  over  his  lo- 
cality ;  and,  from  a  knowledge  of  the  general  distribution 
of  the  meteorological  elements  in  the  ideal  cyclone  or  anti- 
cyclone, he  can  foretell  with  some  degree  of  accuracy  what 
the  coming  weather  will  be.  For  making  local  predictions 
for  the  morrow  from  the  aspect  of  the  sky,  the  evening 
at  about  sunset  is  a  very  favorable  time. 

Observed  Shifting  of  the  Wind. —  In  the  case  of  a 
cyclone  approaching  from  the  west,  if  the  wind  gradually 
changes  from  the  east  around  through  the  north  to  the 
northwest,  then  the  cyclone  center  is  passing  on  the  south 
side  of  the  observer,  and  he  encounters  in  succession  the 
weather  conditions  on  the  northern  or  left  side  of  the 
cyclone ; .  but  if  the  wind  changes  from  the  east  or  south- 
east around  through  the  south  to  the  west  and  northwest, 
then  the  cyclone  center  is  passing  to  the  north  of  the 
observer,  and  he  encounters  in  succession  the  weather 
conditions  on  the  southern  or  right-hand  side  of  the 
cyclone.  In  the  case  of  a  cyclone  approaching  from  the 
south,  when  the  wind  changes  from  northeast  around 
through  the  west  the  observer  is  on  the  left  or  western 
side  of  the  cyclone ;  but  when  the  wind  changes  around 
through  the  east,  the  observer  is  on  the  right  or  eastern 
side  of  the  cyclone. 

When  the  observer's  position  lies  in  the  path  of  the 
center  of  the  cyclone,  the  wind  at  first  remains  steady  in 
direction,  but  almost  ceases  with  the  passage  of  the  center, 
and  immediately  afterwards  blows  from  the  proper  quarter 


2/6  ELEMENTARY   METEOROLOGY 

on  the  following  side  of  the  cyclone,  without  the  gradual 
shifting  of  direction  ordinarily  noticed.  A  rapid  and  great 
fall  in  the  barometric  pressure  indicates  the  near  passage 
of  the  center  of  the  cyclone. 

Introduction  of  Weather  Charts  and  Storm  Warnings.  — 
About  the  middle  of  the  eighteenth  century  it  was  dis- 
covered that  great  atmospheric  disturbances  known  as 
storms  occurred  first  in  the  south  and  the  west  of  the 
American  colonies,  and  moved  in  a  northeasterly  direction 
over  them  ;  but  the  portrayal  of  the  construction  or  mech- 
anism of  these  atmospheric  disturbances  was  not  shown 
until  after  the  first  third  of  the  nineteenth  century,  when, 
by  means  of  charts  showing  the  simultaneous  distribution 
of  the  meteorological  elements  over  a  large '  region  of 
country,  the  existing  conditions  were  brought  out,  and  the 
movement  of  translation  over  a  limited  portion  of  the  earth's 
surface  could  be  followed. 

The  introduction  of  the  telegraph  made  it  possible  to 
collect  meteorological  data  from  a  large  section  of  country 
in  time  to  make  it  of  use  in  following  the  weather  changes 
over  the  whole  region  at  the  time  the  events  are  actually 
taking  place,  and  also  to  transmit  storm  warnings  in  ad- 
vance of  the  approach  of  the  storm.  Systematic  work  of 
this  kind  was  begun  in  a  limited  way  about  the  middle  of 
the  nineteenth  century,  and  during  the  last  quarter  of  the 
century  most  civilized  countries  have  had  regularly  estab- 
lished services  for  carrying  on  a  work  of  this  nature. 

Use  of  Weather  Maps  for  Weather  Predictions.  —  Briefly 
stated,  the  usual  method  of  using  weather  maps  for  pre- 
dicting the  weather  is  as  follows :  The  charted  meteoro- 
logical observations,  and  especially  observations  of  the  air 
pressure,  show  us  the  location,  on  the  map,  of  cyclones  and 
anticyclones,  and  the  accompanying  weather  conditions. 


WEATHER  AND   WEATHER   PREDICTIONS.  277 

These  pass  over  the  country  from  the  west  towards  the 
east  (or  sometimes  from  the  south  towards  the  northeast) ; 
and,  when  we  see  the  rate  at  which  they  are  moving,  it 
can  be  calculated  about  where  they  will  be  at  any  time 
during  the  next  24  or  48  hours. 

Thus  we  follow  the  movement  of  cyclones  and  anticy- 
clones and  their  accompanying  weather  conditions  across 
the  country  in  much  the  same  manner  that  we  can  follow 
the  movements  of  a  railroad  train  if  we  know  its  time 
and  place  of  starting,  and  its  route  and  speed.  But  the 
cyclones  vary  so  much,  in  intensity,  in  the  paths  which 
they  take,  and  in  the  velocity  of  movement,  that  their 
positions  and  conditions  can  usually  be  foretold  only  day 
by  day.  Once  having  fixed  the  position  of  a  cyclone  or 
anticyclone  with  regard  to  any  place,  we  know  the  general 
weather  conditions  at  this  place  as  shown  by  the  distri- 
bution of  the  meteorological  elements  in  cyclones  and 
anticyclones. 

We  have  now  to  see  exactly  how  weather  charts  are 
constructed,  and  then  more  particularly  how  they  are 
used  for  making  weather  predictions. 

Simultaneous  Meteorological  Observations  for  use  on 
weather  maps  are  made  at  certain  hours  of  the  day  (at 
present  in  the  United  States  at  8  A.M.  and  8  P.M.,  75th 
meridian  time)  by  regularly  appointed  observers  at  the 
government  meteorological  observing  stations  distributed 
throughout  the  country.  The  results  of  these  observations 
are  at  once  telegraphed  to  some  central  office,  where 
specially  appointed  'officials  receive  them,  and  by  means 
of  them  make  charts  of  the  condition  of  the  various 
meteorological  elements,  and  the  weather  prevailing  over 
the  whole  region  from  which  such  reports  have  been 
received. 


278  ELEMENTARY  METEOROLOGY. 

There  are  over  a  hundred  observing  stations  in  the  United  States, 
and  perhaps  a  dozen  more  in  Canada,  from  which  complete  reports  of 
this  nature  are  received  by  telegraph  at  the  United  States  Weather 
Bureau  office  at  Washington  twice  a  day  just  after  the  observations 
are  made.  In  England  such  reports  are  received  at  London ;  in 
France,  at  Paris  ;  in  Germany,  at  Hamburg  and  other  cities  ;  in  Austria, 
at  Vienna ;  in  Russia,  at  St.  Petersburg ;  etc. 

Construction  of  Weather  Maps.  —  In  the  construction  of 
weather  maps  there  is  used  as  a  basis  an  outline  geograph- 
ical chart  of  the  whole  region  from  which  meteorological 
data  are  received  by  telegraph. 

Air  Pressure. — The  barometric  pressures  reduced  to 
sea  level  are  written  down  at  the  stations  to  which  these 
belong,  and  then  the  isobaric  lines  are  drawn.  These  full, 
heavily  drawn  lines  (which  are  placed  on  United  States 
weather  charts  at  intervals  of  one  tenth  of  an  inch  air 
pressure)  have  marked  on  each  in  large  figures  the  baro- 
metric pressure  which  they  represent,  and  they  show  at 
a  glance  where  the  cyclonic  and  anticyclonic  areas  are 
located.  The  cyclone  centers  are  then  marked  in  large 
letters  LOW  to  signify  low  barometric  pressure.  The 
anticyclone  centers  are  marked  HIGH  to  signify  high 
barometric  pressure. 

These  isobars  are  the  most  important  feature  of  the  weather  map ; 
and  a  person  skilled  in  the  study  of  such  maps  could  make  good 
weather  predictions  from  a  chart  containing  them  alone,  as  he  is  famil- 
iar with  the  usual  distribution  of  the  meteorological  elements  in  and 
around  cyclones  and  anticyclones. 

Temperature.  —  On  this  same  chart  the  observed  tem- 
peratures are  written  down  at  the  stations  to  which  they 
belong.  Isothermal  lines  are  then  drawn  at  intervals  of 
5°  or  10°  F.  usually.  These  isotherms  are  heavy  dotted 
lines,  and  each  has  marked  on  it  in  large  figures  the 
proper  temperature. 


WEATHER   AND   WEATHER   PREDICTIONS.  279 

These  dotted  lines  show  at  a  glance  the  regions  of  cold  and  warm 
air.  Where  they  bend  towards  the  equator,  we  know  that  the  local 
air  is  relatively  cold ;  and  where  they  bend  towards  the  pole,  the  air 
is  relatively  warm.  On  the  American  maps,  regions  in  which  marked 
changes  of  temperature  have  occurred  during  the  past  24  hours  are  in- 
closed by  a  heavy  dotted  line ;  predicted  cold  waves  are  marked  C.  W. 

Wind,  —  The  wind's  direction  is  indicated  at  each  station 
by  drawing  an  arrow  through  the  center  of  the  station  ;  the 
arrow  flying  with  the  wind.  The  velocity  of  the  wind  is 
sometimes  written  down  in  miles  per  hour  beside  the 
arrow ;  or  lines  may  be  drawn  in  the  tail  of  the  arrow  to 
indicate  feathers,  the  greater  number  of  lines  signifying 
the  greater  wind  velocity.  Wind  velocities  vary  too  greatly 
to  easily  permit  the  drawing  of  lines  of  equal  velocities. 

Moisture.  —  The  aspect  of  the  sky,  and  the  amount  of 
moisture  as  indicated  by  the  degree  of  cloudiness,  are 
shown  on  the  map  by  a  circle  drawn  around  each  station. 
Thus  a  O  is  made  around  the  station  when  the  weather 
is  fair  and  the  sky  is  clear,  and  the  space  within  the  cir- 
cle is  filled  up  as  the  degree  of  cloudiness  increases ;  and 
when  it  is  raining,  the  circle  is  entirely  filled  up. 

In  the  American  maps  a  bar  is  drawn  through  the  circle  ((D) 
when  the  sky  is  half  covered  with  cloud.  A  small  white  center  is 
still  left  (O)  when  the  sky  is  entirely  covered  with  cloud ;  and  the 
circle  is  entirely  filled  up  (•)  when  it  is  raining.  When  it  is  snow- 
ing, several  bars  are  drawn  through  the  open  circle  ((§).  The 
arrows  showing  the  direction  of  the  wind  pass  through  the  centers 
of  these  circles.  Such  symbols  are  subject  to  change. 

Also  on  the  American  charts,  the  regions  in  which  rain  has  fallen 
in  the  past  12  hours  are  shown  by  a  slight  shading  on  the  map.  In 
cases  of  cyclones  with  low  pressure  and  correspondingly  strong  winds 
and  much  rain,  the  previous  path  of  the  cyclone  over  the  chart  is  marked 
by  an  arrow-headed  line,  and  on  it  are  placed  circles  with  a  cross  (-0-) 
to  indicate  the  location  of  the  center  of  the  cyclone  at  the  previous 
times  of  observation,  which  are  shown  by  the  attached  dates. 

WALDO  METEOR.  —  17 


(280) 


(28 1) 


282  ELEMENTARY   METEOROLOGY. 

Such  a  weather  map  shows,  first  of  all,  the  absolute 
and  relative  positions  of  the  areas  of  high  barometric  pres- 
sure (anticyclones)  and  low  barometric  pressure  (cyclones), 
which,  in  the  main,  control  the  characteristics  of  the 
weather  in  our  latitudes.  It  shows  the  area  dominated 
by  the  weather  conditions  pertaining  to  each.  It  also 
shows  the  magnitude  of  the  excess  and  deficiency  of 
pressure  on  which  the  intensity  of  the  resulting  condi- 
tions depend.  Tabular  data  accompany  most  maps. 

In  order  to  present  the  idea  of  weather  charts  more  clearly,  there 
are  given  here  (Figs.  87  and  88)  two  actual  maps  for  the  United  States, 
showing  the  simultaneous  weather  conditions  in  midsummer  and  again 
in  midwinter,  the  two  extreme  seasons  of  the  year.  Very  pro- 
nounced specimens  of  the  winter  and  summer  types  of  weather  have 
been  selected  in  order  to  bring  out  their  characteristics.  These  maps 
will  be  comprehended  from  the  foregoing  details  concerning  their  con- 
struction, and  they  should  be  carefully  studied. 

Scope  of  Weather  Predictions,  —  Weather  predictions 
are  made  either  for  a  single  place  or  for  a  whole  region ; 
and  they  are  made  for  a  definite  time,  say  for  12,  24, 
or  36  hours  in  advance,  or  else  the  weather  changes  are 
designated  which  are  gradually  to  occur  during  the  com- 
ing 12,  24,  or  36  hours, 

It  is  best  at  first  to  select  some  definite  point  for  which  the  weather 
is  to  be  predicted ;  and  the  best  one  will  ordinarily  be  that  at  which 
the  person  making  the  prediction  lives,  because  it  will  then  become  a 
matter  of  practical  interest.  After  some  experience  has  been  thus 
acquired,  predictions  can  be  made  for  some  distant  point,  and  still 
later  for  a  whole  region. 

Irregular  Movements  of  Cyclones  and  Anticyclones.  — 

These  atmospheric  disturbances  move  (in  our  latitudes)  gen- 
erally in  an  easterly  direction,  and  the  probability  is  that 
the  velocity  of  translation  for  a  few  hours  preceding  any 


WEATHER  AND   WEATHER   PREDICTIONS.  283 

definite  time  will  remain  the  same  for  a  few  hours  after 
this  time ;  that  is,  if  the  cyclone  or  anticyclone  has  moved 
at  the  rate  of  25  miles  per  hour  during  the  past  12  or  24 
hours,  then  it  will  probably  continue  so  to  move  during 
the  next  period  of  like  length. 

Some  weather  predictors  consider  it  best  to  take  as  the  probable 
future  velocity  the  average  velocity  of  the  cyclones,  or  some  compro- 
mise between  it  and  the  velocity  during  the  hours  just  passed. 

Cyclones  entering  our  continent  from  the  west,  and  cross- 
ing it,  frequently  move  at  first  in  a  southeasterly  direction, 
and  then  in  a  northeasterly  direction ;  and  those  which 
enter  the  continent  from  the  south  or  southeast  usually 
move  off  in  a  northeasterly  direction.  Cyclones  have  a 
tendency  to  move  slightly  spirally  around  to  the  right  of 
areas  of  low  temperature,  and  they  have  a  tendency  to 
drift  in  the  direction  of  the  main  atmospheric  motion,  as 
shown  by  the  direction  of  the  higher  clouds,  and  to  move 
in  the  direction  of  greatest  rainfall. 

When  areas  of  high  barometric  pressure  (anticyclones) 
and  of  low  pressure  (cyclones)  occur  simultaneously  within  a 
limited  region,  as  on  the  same  weather  map,  they  seem  to 
mutually  affect  each  other's  condition,  behavior,  and  move- 
ments. When  an  anticyclone  is  on  the  north  or  east  side 
of  a  cyclonic  area,  there  is  a  tendency  for  the  latter  to  be 
retarded  and  deflected  towards  the  north ;  but  if  the  anti- 
cyclone is  to  the  south  or  southeast  of  the  cyclone,  the 
movement  seems  to  be  accelerated.  Where  one  cyclone 
follows  another  closely,  the  one  in  the  rear  seems  to  have 
a  retarding  and  deflecting  effect  on  the  one  in  front 

After  having  decided  on  the  probable  future  path  of  the 
cyclones  and  anticyclones,  and  their  accompanying  condi- 
tions shown  on  the  weather  map,  the  making  of  a  weather 


284  ELEMENTARY   METEOROLOGY. 

prediction  consists  in  following  out  the  changes  which  will 
take  place  when  the  existing  conditions  are  moved  along  to 
the  places  at  which  they  will  be  if  the  movement  takes 
place  as  expected.  All  of  the  existing  observed  conditions 
may  be  just  moved  along  over  the  stationary  map  in  the 
most  frequented  path,  and  at  the  average  rate  of  move- 
ment of  the  cyclones,  and  we  can  see  on  the  map  where 
these  conditions  will  fall. 

The  conditions  actually  do  change,  but  it  requires  great 
skill  on  the  part  of  the  predictor  to  foretell  these  changes. 

Variations  in  Conditions  on  Weather  Charts.  —  On  the 
first  consideration  of  a  weather  map,  and  recognizing  that  it 
is  made  up  of  areas  of  high  and  low  air  pressure,  it  might 
be  quite  natural  to  think  that  in  short  intervals  of  time,  say 
every  few  days,  the  maps  would  repeat  themselves,  and 
that  many  would  be  found  quite  identical.  But  this  is  not 
the  case,  for  a  great  variety  of  combinations  exists,  and  two 
maps  are  seldom  alike.  Still  it  is  possible  to  class  weather 
maps  in  a  general  way  according  to  certain  types,  but  the 
number  of  these  types  is  very  great.  Probably  over  100 
typical  forms  would  be  necessary  for  such  classification ; 
some  one  of  them  might  occur  only  at  intervals  of  several 
years,  while  others  would  occur  several  times  in  a  year. 

It  has  also  been  found  that  the  behavior  of  the  cyclones 
and  the  anticyclones  is  not  the  same  on  maps  which  appear 
to  be  almost  exactly  alike ;  so  that,  because  they  follow 
certain  movements  in  one  case,  it  is  by  no  means  certain 
or  even  probable  that  the  same  will  occur  for  other  cases 
of  the  same  type  of  form  and  arrangement.  It  is  this 
variability  under  apparently  like  circumstances  which  pre- 
cludes a  much  further  advance  in  accuracy  of  weather 
predictions  by  the  methods  now  adopted. 

Weather  Predictions  for  Different  Regions.  —  It  must  be 


WEATHER  AND   WEATHER   PREDICTIONS.  285 

assumed,  in  the  case  of  general  weather  predictions  which 
are  made  for  a  whole  continent  from  a  single  point  (as  at 
Washington),  that  there  will  not  be  much  change  in  the 
distribution  of  the  weather  elements  in  the  cyclonic  and 
anticyclonic  areas  as  they  traverse  the  continent ;  but  they 
are  as  a  fact  continually  undergoing  changes,  which  can 
seldom  be  anticipated  at  the  proper  time.  As  the  cyclones 
and  anticyclones  move  over  the  land,  new  air  is  brought 
into  the  circulation  ;  and  it  makes  a  great  difference  in  the 
future  conditions  within  the  areas  whether  the  air  thus 
drawn  in  has  a  constant  or  varying  supply  of  moisture. 

Any  portion  of  the  cyclonic  area,  as  for  instance  the  southeast  quad- 
rant, may  have  certain  definite  conditions  when  the  southwest  wind 
which  supplies  it  with  air  comes  from  a  dry  region  (as,  for  instance,  in 
Colorado)  ;  but  let  the  area  progress  eastward,  and  the  air  be  drawn 
from  a  moister  region  (say,  the  Ohio  River  valley),  and  the  weather 
would  not  be  of  such  a  nature  that  a  prediction  for  the  dry  region 
(Colorado)  would  fit  the  existing  conditions  in  the  new  locality.  Let 
the  storm  progress  still  farther,  so  that  the  southeast  quadrant  would 
be  on  a  warm  moist  coast  (say,  on  the  southern  coast  of  New  England), 
then  the  warm  moist  air  entering  it  (from  the  Gulf  Stream  region  of 
the  Atlantic  Ocean)  would  cause  still  other  conditions  to  arise. 

The  meteorological  and  topographical  characteristics  of 
different  regions  must  be  studied  very  closely  by  the  person 
making  predictions  for  them. 

Combination  of  Weather  Map  and  Local  Meteorological 
Conditions.  —  It  is  now  a  recognized  fact  that  a  person  on 
the  field  of  any  region  can,  with  the  aid  of  weather  maps, 
make  more  accurate  weather  predictions  for  that  region 
than  a  person  at  a  distance,  who  is  unacquainted  with  the 
local  characteristic  weather,  and  relies  entirely  on  the 
weather  map.  In  addition  to  the  general  conditions  of  en- 
vironment, there  are  many  local  peculiarities  which  cannot 


286  ELEMENTARY  METEOROLOGY. 

be  taken  into  account  in  telegraphic  reports,  and  which 
greatly  affect  the  weather  conditions  at  a  place.  It  seems 
best,  therefore,  to  have,  for  each  geographical  section 
where  the  conditions  are  similar,  a  local  weather  predictor 
who  shall  receive  telegraphic  weather  reports,  and  have  the 
use  of  maps  constructed  therefrom,  and  who  shall  modify 
the  predictions  based  on  this  by  the  local  conditions. 

This  method  is  now  adopted  by  the  United  States  Weather  Bureau ; 
and  it  seems  to  be  more  successful  than  the  older  method,  where 
one  predictor  at  Washington  had  to  furnish  from  the  weather  map 
alone  the  forecasts  for  all  the  various  regions  of  the  whole  country. 
This  combined  method  has  been  in  use  in  Europe  since  the  introduc- 
tion of  telegraphic  weather  services.  There  the  various  countries  unite 
in  the  interchange  of  the  observed  data,  so  that  the  predictor  in  each 
country  can  have  available  the  whole  continental  distribution  of  mete- 
orological data ;  but  the  official  predictor  for  each  country  makes  the 
weather  forecasts  for  his  own  country  alone.  The  form  of  weather 
maps  varies  in  the  different  lands,  although  the  data  used  in  all  are 
about  the  same.  This  makes  the  predictions  in  each  case  (except  for 
Russia)  apply  to  a  limited  region  only ;  and  this  is  even  more  necessary 
in  Europe  than  in  America,  for  in  the  former  the  weather  predictions 
are  much  more  difficult  to  make,  on  account  of  both  the  variability  of 
the  paths  of  the  cyclonic  areas  in  high  latitudes,  and  the  fact  that  the 
atmospheric  disturbances  come  from  the  ocean  to  the  westward,  whence 
telegraphic  communications  are  not  available. 

Thunderstorm  Predictions.  —  Thunderstorms  and  torna- 
does belong  to  that  class  of  local  meteorological  phe- 
nomena that  may  be  predicted  for  a  general  region,  but  it 
is  impossible  to  predict  their  formation  at  any  particular 
place.  When,  however,  they  have  once  formed,  a  knowl- 
edge of  their  presence  can  be  transmitted  by  telegraph  or 
telephone  to  points  which  lie  to  the  eastward  or  northeast 
of  them,  and  which  lie  in  their  probable  paths.  Undoubt- 
edly more  harm  than  good  would  be  done  by  predictions  of 


WEATHER  AND  WEATHER   PREDICTIONS.  287 

tornadoes  for  whole  regions  in  which  they  are  likely  to 
occur.  In  the  case  of  thunderstorms,  however,  they  trav- 
erse such  a  great  section  of  country,  that,  when  it  is  found 
that  they  are  active  in  a  certain  section  of  a  cyclonic  area, 
their  probable  occurrence  in  the  same  relative  position  to 
this  area  may  be  predicted  in  its  farther  course ;  but  for 
any  region  lying  in  their  path  nothing  more  definite  can 
be  said  than  that  local  (here  and  there)  thunderstorms 
may  be  expected. 

Prediction  of  Cold  Waves.  —  By  cold  wave  is  meant  the 
fall  of  temperature  over  a  large  area  from  one  day  to 
the  next;  and  this  change  must  be  more  than  temporary 
and  local,  for  the  fall  of  temperature  which  occurs  during 
a  thunderstorm  would  not  be  called  a  cold  wave.  Cold 
waves  are  nearly  always  connected  with  cyclonic  and 
anticyclonic  disturbances.  They  almost  invariably  occur 
to  the  west  of  a  cyclonic  and  to  the  east  of  an  anticy- 
clonic area.  While  cyclonic  areas  usually  move  easterly 
or  northeasterly,  cold  waves  generally  advance  from  day 
to  day  in  a  southerly  or  southeasterly  direction.  The 
greatest  temperature  fall  during  24  hours  is  usually  quite 
near  to  the  center  of  low  pressure  in  the  cyclone,  and  is 
most  likely  to  occur  a  little  to  the  south  or  west  of  this 
center.  The  direction  of  most  rapid  change  in  the  tem- 
peratures within  a  cold  wave  is  usually  towards  the  north- 
west of  the  center  of  the  cyclone. 

A  change  of  60°  F.  in  the  temperature  from  one  day  to  the  next 
(24  hours)  was  observed  in  the  United  States  only  twice  from  1880  to 
1890 ;  and  a  change  of  from  50°  to  60°  F.,  on  but  16  days  in  10  years. 

Cold-wave  Areas,  which  may  in  extreme  cases  cover 
1,000,000  square  miles,  are  usually  elliptical,  with  the  long 
axis  extending  from  southwest  to  northeast,  and  parallel  to 


288  ELEMENTARY   METEOROLOGY, 

the  long  axis  of  the  cyclone  in  front  of  it.  While  the 
length  and  breadth  are  sometimes  nearly  equal,  yet  the 
one  axis  may  be  eight  times  as  long  as  the  other. 

In  the  United  States  Weather  Bureau,  the  name  cold  wave  is 
given  when  the  following  temperature  changes  take  place  in  24  hours 
(for  instance,  from  8  A.M.  one  day  to  8  A.M.  the  next  day):  in  the 
northwestern  States,  from  Minnesota  to  Montana,  a  fall  of  20°  F., 
and  temperature  going  as  low  as  32°  F. ;  in  the  central  region,  extend- 
ing from  Colorado  to  Maine,  and  as  far  south  as  Tennessee,  a  fall  of 
1 8°  F.,  and  temperature  going  down  to  34°  F. ;  in  the  southern  United 
States,  a  fall  of  16°  F.,  and  temperature  going  down  to  36°  F.  The 
approach  of  a  cold  wave  is  announced  by  the  United  States  Weather 
Bureau  by  the  display  of  a  square  white  flag  with  a  square  black  center. 

Prediction  of  Hot  Waves.  —  Hot  waves  may  be  referred 
to  their  three  possible  causes  :  — 

1.  The  ground  and  the  lower  layers  of  air  may  be  heated 
to  an  excessive  degree  by  the  continuous  action  of  the 
solar  rays  during  successive  clear  days ;  and  this  accumula- 
tion of  heat  becomes  greatest  when  the  days  are  clear  and 
the  nights  are  clouded.     These  hot  waves  are  of  local  char- 
acter, but  may  be  very  extensive. 

2.  The  air  of  vertical   currents  may  have  its  temper- 
ature raised  by  the  foe/in  process.     Such  conditions  may 
be  expected  on  the  leeward  side  of  mountain  ranges,  where 
the  air  on  the  windward  side  is  moist.     These  hot  waves 
are  usually  confined  to  narrow  bands  parallel  to  the  moun- 
tains.    Similar  hot  waves  also  occur  in  anticyclones,  where 
the  descending  current  is  fed  by  air  which  has  moved  up- 
wards in  a  cyclone,  and  been  deprived  of  a  portion  of  its 
moisture  before  it  descends  again.     Such  hot  waves  follow 
the  course  of  the  anticyclones. 

3.  Warm    air   may  be    blown    from  warmer   to    colder 
regions  by  the  continuation  of  the  winds  in  the  proper 


WEATHER  AND   WEATHER   PREDICTIONS.  289 

direction  during  an  interval  of  several  days.  Such  heated 
conditions  of  the  air  may  be  expected,  in  middle  latitudes, 
just  preceding  the  great  cyclones  and  following  the  anticy- 
clones; for  in  both  of  these  cases  the  air  is  blown  from 
lower  and  warmer  to  higher  and  colder  latitudes  some- 
times for  days  in  succession.  These  hot  waves  are  of 
greatest  extent. 

Predictions  of  Hurricanes.  —  In  the  hurricanes  which 
come  from  the  West  Indies,  and  skirt  the  eastern  coast  of 
the  United  States,  the  storm  center  sometimes  moves  along 
over  the  ocean,  very  frequently  follows  the  Gulf  Stream, 
but  sometimes  penetrates  the  Gulf  of  Mexico ;  or,  pursuing 
a  course  somewhat  inland,  it  ravages  the  whole  Atlantic 
seaboard.  Such  storms  require  special  predictions. 

In  the  incipient  stage  of  the  hurricane  there  is  but  a  small  area 
affected  by  it,  and  at  this  stage  no  characteristic  signs  of  its  approach 
precede  it  at  any  great  distance.  It  is  therefore  very  necessary  for  the 
official  weather  predictor  at  Washington  to  receive  as  early  as  possible  a 
telegram  from  the  West  Indian  Islands,  announcing  the  existence  of 
the  hurricane ;  and  additional  telegrams  should  be  received  stating  its 
progressive  movement.  The  storm  center  moves  so  slowly  that  it  may 
be  several  days  after  its  announcement  before  it  reaches  the  coast  waters 
of  the  United  States  ;  and  very  frequently  the  wave  movement  imparted 
to  the  ocean  water  by  its  violence  will  reach  the  shores  of  the  United 
States  before  the  storm  itself  is  noticeable  on  the  weather  maps.  Often 
the  passage  of  such  a  hurricane  far  out  at  sea  is  made  manifest  by  the 
waves  which  it  causes  to  lash  the  coast  hundreds  of  miles  from  the 
storm  center,  while  no  trace  of  the  latter  is  visible  on  the  land  weather 
maps. 

The  successful  predictions  of  the  movement  of  the  hurricanes  after 
they  have  entered  the  region  covered  by  the  weather  maps,  is  a  matter  of 
much  difficulty,  for  their  exact  direction  of  motion  is  too  uncertain,  and 
their  rate  of  progress  is  too  variable,  to  foretell  the  exact  location  of  the 
storm's  center.  The  warnings  which  navigators,  especially  of  small 
coasting  vessels,  receive  concerning  these  storms  is  of  incalculable  value 
to  thenio 


2QO  ELEMENTARY   METEOROLOGY, 

When  the  East  Indian  typhoons  approach  the  Chinese  and  Japanese 
coasts,  almost  unheralded  except  at  telegraphic  points,  the  loss  of  life 
and  damage  to  shipping  is  very  great. 

Predictions  of  River  Floods.  —  During  a  river  flood  there 
is  a  period  during  which  the  water  rises,  another  when  it 
is  at  its  greatest  height,  and  a  third  when  it  falls.  A  flood 
may  then  be  viewed  as  a  progressive  wave  which  has  the 
culminating  point  of  the  flood  for  its  crest.  The  movement 
of  this  crest  is  followed  on  the  map  by  the  predictor  as  it 
proceeds  down  the  river  valley  ;  and  the  time  of  its  appear- 
ance is  predicted  for  points  below  by  means  of  its  speed  as 
determined  by  the  velocity ,  of  the  current  at  the  highest 
point  of  the  flood.  Such  predictions  can  be  made  usually 
several  days  in  advance  for  places  lying  on  the  lower  part 
of  the  river  course.  When  a  river  is  fed  by  several  branches, 
it  is  necessary  to  predict  the  flood  conditions  of  each  effec- 
tive branch,  and  then  combine  these  predictions  in  order 
to  determine  the  flood  conditions  for  the  main  stream 
below  the  point  of  union  of  the  branches. 

Frost  Warnings  (Cold- wave  Frosts).  —  When  the  tem- 
perature is  high  and  a  cold  wave  makes  it  fall  considerably 
below  the  freezing  point,  then  one  kind  of  a  frost  occurs ; 
and  it  will  be  very  general  and  of  a  wide  distribution,  and 
may  begin  day  or  night.  When  the  air  is  dry,  and  the 
temperature  does  not  descend  below  the  dew-point  tem- 
perature, as  in  many  cases  of  cold  waves,  little  damage  is 
done  by  the  cold,  even  if  the  temperature  goes  below  the 
freezing  point  of  water ;  but  when  the  dew-point  is  high, 
and  dew  is  deposited  and  then  freezes,  the  damage  to  plant 
growth  may  be  very  great,  even  for  a  temperature  at  or 
just  a  little  below  the  freezing  point  of  water. 

Night  Frosts.  —  The  more  local  frosts  are  those  which, 
under  certain  conditions,  may  occur  during  the  usual  mini- 


WEATHER   AND   WEATHER   PREDICTIONS.  29 1 

mum  temperature  at  night.  Briefly  stated,  it  may  be  said 
that  night  frost  is  to  be  feared  when  observations  of  the 
temperature  and  humidity  show  that  the  dew-point  lies 
below  the  freezing  point,  and  it  is  expected  that  the  tem- 
perature will  descend  below  the  dew-point. 

Storm  Signals.  —  In  all  civilized  countries  where  weather 
services  are  maintained,  special  signals  announcing  the 
approach  of  storms  are  displayed  at  seaports  for  the  bene- 
fit of  mariners.  These  signals  are  usually  flags  by  day, 
and  lanterns  by  night. 

Distribution  of  Weather  Predictions  (especially  in  the 
United  States).  —  After  the  official  government  weather 
predictor  has  decided  on  the  probable  weather  predictions 
for  the  next  24  or  36  hours,  this  forecast  must  be  placed 
before  the  public.  This  is  accomplished  in  a  number 
of  ways.  Daily  weather  maps  such  as  have  been  de- 
scribed are  mailed  from  various  centers  to  those  places 
which  can  be  reached  in  time  to  be  of  any  use,  and  these 
are  displayed  in  public  buildings,  such  as  post  offices. 

The  telegraph  and  telephone  services  are  more  and 
more  used  for  distributing  weather  predictions  to  distant 
and  local  points,  and  especially  in  transmitting  them  to 
all  of  the  great  daily  newspapers  of  the  country.  It  is 
through  these  daily  papers  that  the  widest-spread  knowl- 
edge of  the  weather  forecasts  is  obtained. 

Use  is  also  made  of  the  frequent  railroad  trains  in  some  regions  to 
display  on  them  flags  which  shall  communicate  to  the  inhabitants  near 
the  railroad  the  coming  weather  conditions ;  and  the  railroad  telegraph 
conveys  the  forecasts  to  each  station  on  the  route,  where  they  are 
posted  up  for  public  information.  The  press  telegraphic  organizations 
transmit  the  predictions  to  the  city  newspapers. 

Accuracy  of  Weather  Predictions. -- The  accuracy  of 
weather  predictions  is  measured  in  percentage  of  their 


2Q2  ELEMENTARY   METEOROLOGY. 

complete  success.  It  is  estimated  that,  on  the  average, 
from  80%  to  85%  of  weather  predictions  are  successful. 
The  accuracy  varies  considerably,  not  only  for  the  differ- 
ent meteorological  elements,  but  also  for  different  regions 
of  a  country.  Those  regions  are  the  most  difficult  to  pre- 
dict for  which  have  the  greatest  and  the  most  sudden 
weather  changes.  It  is  easier  to  predict  the  weather 
during  the  season  of  settled  weather  or  least  rain  than 
when  rainfall  is  most  frequent.  It  is  also  easier  to  predict 
temperature  changes  within  certain  fixed  limits  in  the 
summer  than  in  the  winter. 

The  predictions  of  wind  velocities  and  cold  waves  are  about  the  least 
successful,  averaging  only  about  60%;  while  the  wind  direction  is  the 
most  successful,  averaging  over  90  %.  The  weather  predictions  for  the 
California  coast  are  the  most  successful  of  any  in  the  United  States. 

Long-range  Weather  Predictions.  —  While  it  is  possible 
in  our  latitudes  to  make  predictions  concerning  the  weather 
for  the  next  24  or  even  36  hours,  and  in  special  cases  even 
longer,  yet,  in  general,  the  making  of  definite  forecasts  for 
successive  days,  or  for  any  specified  date  several  days 
ahead,  cannot  be  successfully  accomplished.  All  long-range 
predictions,  such  as  for  a  coming  season,  are  groundless ; 
and  the  prediction  of  any  great  storms  for  certain  dates  far 
in  the  future  is  utter  nonsense,  as  no  one  can  foretell  the 
appearance  of  a  particular  storm  until  it  actually  begins  to 
form.  It  is  possible,  however,  to  state  the  average  weather 
conditions  which  have  been  found  to  exist  at  different 
periods  during  the  year. 


CHAPTER    XIL 
CLIMATE. 

Climatic  Conditions.  —  The  term  climate  signifies  the 
aggregate  or  average  of  meteorological  conditions.  The 
climate  of  a  place  can  be  determined  only  by  a  series  of 
observations  of  the  meteorological  elements,  carried  on 
through  a  period  long  enough  to  give  the  average  con- 
ditions, freed  from  the  irregularities  due  to  accidental 
weather  conditions,  and  to  determine  the  average  and 
possible  extreme  departures  from  those  average  conditions. 

Weather  is  but  a  phase  of  climate  extending  over  a 
not  too  long  definite  interval  of  time. 

We  speak  of  the  meteorological  conditions  for  a  single  year,  or  a 
season,  or  a  day,  as  weather.  Thus  we  say,  "  last  summer  the  weather 
was  wet,"  and  not,  "the  climate  was  wet."  We  could  not  say  that  the 
climate  was  wet  during  the  summer,  unless  "it  was  so  during  the  majority 
of  summers.  We  should  speak  of  the  annual  or  seasonal  climate,  and 
perhaps  also  for  the  months,  but  not  for  shorter  periods. 

Climatology  is  the  science  of  climate,  and  its  object  is 
to  present  the  average  joint  action  (and  possible  variations 
therefrom)  of  the  meteorological  elements  which  pertain 
to  climate  at  the  different  places  on  the  earth's  surface. 
Climatology  is  therefore  mostly  descriptive.  The  impor- 
tant climatological  elements  are  the  heat,  movement,  and 
moisture  of  the  air  ;  precipitation ;  evaporation ;  cloudiness ; 
and  solar  radiation. 

293 


294  ELEMENTARY   METEOROLOGY. 

The  barometric  pressure  is  of  the  greatest  importance  in  influencing 
the  distribution  of  the  climatological  elements ;  but  it  is  only  in  the 
limited  regions  of  great  altitudes  that  it  can  be  considered  one  of 
them,  and  then  chiefly  on  account  of  its  effects  on  animal  organisms, 
due  to  the  decrease  in  air  density. 

In  studying  climate,  we  must,  then,  study  the  conditions 
and  relations  arising  from  the  combination  of  the  average 
conditions  of  the  above-mentioned  elements,  which  have 
been  studied  separately  in  the  earlier  part  of  this  book. 

The  relative  importance  of  the  climatological  elements, 
and  the  data  usually  required  for  each,  are  as  follows  :  — 

Temperature. 

1.  The  monthly  and  annual  mean  air  temperature. 

2.  The  amount  of  the  daily  temperature  oscillation  in 
the  single  months. 

3.  The  average  and  absolute  monthly  and  annual  tem- 
perature extremes. 

4.  The  average  and  extreme  dates  when  ice  forms  in  the 
spring  and  fall,  and  the  number  of  days  free  from  ice. 

5.  The  average  variability  of  the  daily  temperature  for 
each  month  and  the  year. 

It  is  desirable,  also,  to  have  the  average  monthly  and 
annual  temperatures  at  some  morning,  some  midday,  and 
some  evening  hour,  say  7  A.M.,  2  P.M.,  and  9  P.M. 

Moisture  and  Precipitation  (for  each  month  and  the  year). 

1.  The  average  relative  humidity. 

2.  The  average  amount  of  precipitation. 

3.  The  average  number  of  days  with  precipitation. 

4.  The  average  intensity  of  rainfall. 

5.  The  average  probability  of  rainfall. 
Cloud  (for  each  month  and  the  year). 

1.  The  average  degree  of  cloudiness. 

2.  The  average  number  of  hours  with  sunshine. 


CLIMATE.  295 

Wind  (for  each  month  and  the  year). 

I0    The  average  wind  velocity  or  force. 

2.  The  average  number  of  times  the  wind  blows  from 
each  of  the  eight  points  of  the  compass. 

Evaporation.  —  It  is  desirable  to  obtain  the  average 
monthly  amount  of  water  evaporated  both  from  a  free  water 
surface  exposed  to  the  sun  and  all  weathers,  and  from  a 
shaded  water  surface  ;  but  these  are  very  seldom  measured. 

Solar  Climate. — The  ideal  solar  climate  is  that  which 
would  exist  if  the  sun's  rays  reached  a  homogeneous  earth 
without  an  atmosphere  ;  that  is,  an  earth  with  a  smooth 
surface  all  land  or  all  water.  The  solar  climate  would 
have  a  distribution  varying  with  the  latitude  from  the 
equator  to  the  pole,  but  would  be  the  same  at  all  points 
on  a  given  parallel  of  latitude.  There  would  exist  climatic 
zones  extending  around  the  earth,  and  following  the  paral- 
lels of  latitude. 

The  Telluric  or  Physical  Climate  of  the  earth  is  the  solar 
climate  modified  by  the  atmospheric  conditions,  and  the 
existing  distribution  of  land  and  water  surface  of  the  earth. 
The  most  important  disturbing  causes  are  the  unequal 
distribution  of  land  and  water  surfaces,  and  the  different 
elevations  of  the  land  surface  above  the  sea  level.  These 
give  us  the  three  chief  forms  of  climate  of  telluric  origin  : 
i.  The  land  or  continental  climate.  2.  The  water  or 
oceanic  climate,  3.  The  mountain  climate. 

The  fact  that  the  parallels  of  latitude  run  partly  over  a 
land  and  partly  over  a  water  surface  causes  differences  of 
climate  in  an  east-westerly  direction,  transverse  to  those 
of  the  solar  climate.  These  east-westerly  differences  of 
climate  are  reenforced  by  the  transfer  of  equatorial  heat 
towards  the  pole,  and  of  polar  cold  towards  the  equator,  by 
the  currents  of  air  and  water. 

WALDO  METEOR. —  1 8 


296  ELEMENTARY   METEOROLOGY. 

The  Land  or  Continental  Climate  is  characterized  by  great 
extremes  of  temperature,  —  relatively  high  temperatures^ 
during   the    summer    (or   during   the   daytime),    and    low 
temperatures    during  the   winter    (or   during   the    night). 
The   humidity,  cloud,  and  rainfall  are  deficient. 

The  Water  or  Oceanic  Climate  is  characterized  by  slight 
temperature  extremes  :  the  temperatures  remain  relatively 
low  during  the  summer  (or  during  the  daytime),  and  rela- 
tively high  during  the  winter  (or  during  the  night).  The 
humidity  and  cloud  are  excessive,  and  rainfall  ample. 

Continental  and  Oceanic  Climates  are  best  considered  to- 
gether, in  order  to  compare  the  two  extreme  conditions. 

The  Temperature.  —  When  the  same  amount  of  heat  falls 
on  land  and  water  surfaces,  the  temperature  of  the  land  is 
raised  nearly  twice  as  many  degrees  as  the  water.  The  land, 
then,  in  summer  and  in  the  daytime,  warms  the  air  above 
it  more  quickly  than  does  the  water  surface,  but  the  latter 
gives  up  to  the  air  above  it,  by  evaporation,  more  moisture 
than  does  the  land  surface ;  and  when  this  moisture  is  con- 
densed in  the  higher  air  layers,  it  gives  out  heat  to  them. 

At  night  and  in  winter,  however,  the  land  cools  more 
quickly  than  the  water,  and  so  the  air  temperatures  over 
the  water  do  not  fall  so  low  as  those  over  the  land.  The 
greater  moisture  and  cloudiness  of  the  air  over  the  water 
also  prevent  the  loss  of  heat  as  rapidly  as  over  the  land. 
The  temperature  differences  over  a  land  surface  in  summer 
and  winter  are  intensified  by  the  decrease  of  cloudiness 
towards  the  interior  of  a  continent,  and  these  differences 
over  a  water  surface  are  lessened  by  the  greater  cloudiness 
over  the  oceans.  In  higher  latitudes  a  less  degree  of 
cloudiness  lowers  the  winter  temperatures  and  raises  the 
summer  temperatures.  In  lower  latitudes  a  decrease  of 
cloudiness  increases  the  temperature* 


CLIMATE.  297 

At  about  latitude  40°,  both  north  and  south,  the  land 
and  water  surfaces  have  nearly  equal  temperatures.  On 
the  equatorial  side  of  this  parallel  the  land  surface,  and 
on  the  polar  side  the  water  surface,  is  the  warmer,  on 
the  average,  for  the  year. 

At  the  equator  the  temperature  for  a  land  surface  would  be  about 
113°  F.,  and  for  a  water  surface  72°  F. ;  at  latitude  45°,  both  land  and 
water  would  have  a  temperature  of  about  50°  F. ;  while  at  the  pole  the 
temperatures  would  be,  for  land  —25°  F.,  and  for  water  12°  F. 

The  average  temperature  of  a  parallel  can  be  considered 
as  dependent  on  the  latitude  and  on  the  relative  amounts 
of  land  and  water. 

The  Wind. — The  differences  in  temperature  between 
the  land  and  water  surfaces  give  rise  to  the  daily  land  and 
sea  breezes  on  the  coast,  and  to  the  seasonal  monsoon  winds 
extending  from  the  interior  of  the  continents  far  out  over 
the  oceans.  The  winds  belonging  to  the  cyclonic  and 
anticyclonic  circulation  are  not  influenced  very  much  by 
the  land  and  sea  surfaces,  except  that  the  winds  become 
stronger  over  the  water  surface  because  the  friction  is  less 
than  over  the  land. 

The  most  important  climatic  effects  of  the  wind  are  its 
transference  of  moisture  from  the  ocean  to  the  land  sur- 
face ;  of  heated  air  to  colder  regions,  and  cold  air  to  warmer 
regions ;  and  of  air  from  higher  to  lower,  and  lower  to  higher 
altitudes,  thereby  causing  adiabatic  heating  and  cooling. 

The  flow  of  air  between  the  continents  and  oceans  is 
much  more  general  and  also  more  rapid  in  winter  than  in 
summer,  because  the  continents  are  cooled  more  in  winter 
than  they  are  warmed  in  summer.  The  cold  winds  blow 
from  the  continents  in  winter,  and  towards  them  in 
summer. 


298  ELEMENTARY  METEOROLOGY. 

The  windward  coasts  of  the  continent  are  those  towards 
which  the  wind  blows  from  the  ocean,  and  they  have  an 
oceanic  climate.  The  leeward  coasts  of  the  continent  are 
those  from  which  the  wind  blows  towards  the  ocean,  and 
they  have  a  continental  climate.  The  region  to  the  lee- 
ward of  isolated  large  bodies  of  water,  like  the  American 
Great  Lakes,  is  warmer  in  winter  and  cooler  in  summer 
than  the  region  to  the  windward  ;  because  the  water 
surface  is  cooler  in  summer  and  warmer  in  winter  than 
the  land,  and  the  temperature  of  the  air  which  approaches 
from  the  windward  of  a  lake  becomes  tempered  by  the 
water  before  it  reaches  the  land  on  the  leeward  side  of 
the  lake. 

The  Moisture.  — The  moisture  is  transferred  by  the  winds 
from  the  oceans  to  the  continents.  The  windward  coasts 
or  sides  of  the  continents  are  therefore  moister  than  the 
leeward  coasts  ;  because  on  the  windward  side  moist  air 
is  received  from  the  oceans,  while  on  the  leeward  side  the 
air  comes  from  the  interior  of  the  continent,  where  mois- 
ture is  taken  from  the  air  more  rapidly  than  it  is  added. 

The  flatter  and  lower  the  surface  of  the  land,  the  farther  inland  does 
the  air  penetrate  towards  the  interior  of  a  continent  before  it  loses  its 
excessive  moisture.  Where  mountain  ranges  or  other  elevated  lands 
lie  in  the  path  of  the  moisture-laden  air,  much  of  the  moisture  con- 
denses on  the  windward  side,  and  causes  a  wet  climate,  while  on  the 
leeward  side  the  climate  is  dry.  This  is  most  marked  when  the  moun- 
tains lie  close  to  the  ocean,  so  that  precipitation  occurs  before  a  large 
portion  of  the  moisture  has  gradually  been  lost  while  passing  over  a 
large  extent  of  land. 

Climatic  Effects  of  the  Oceanic  Circulation. — The  great 
currents  which  exist  in  the  oceans,  by  which  the  warm 
water  of  the  equatorial  regions  is  carried  towards  the  pole, 
and  the  colder  water  is  transferred  from  the  polar  regions 


CLIMATE.  299 

towards  the  equator,  very  greatly  modify  the  climatic  con- 
ditions over  the  oceans  and  along  coast  lands.  The  warm 
current  heats  the  air  above  it,  and  increases  its  capacity 
for  moisture ;  and  the  cold  current  cools  the  air  above  it, 
and  decreases  its  capacity  for  moisture  ;  and  the  air  thus 
heated  or  cooled  is  blown  by  the  prevailing  winds  on  to 
the  continents,  or  distributed  over  the  adjacent  ocean  sur- 
face. The  temperature  of  the  water  is  communicated 
to  the  land,  only  when  the  wind  blows  from  the  water 
towards  the  land. 

Effect  of  Vegetation,  and  especially  Forests,  on  Climate. 
Local  Effects.  Temperature  of  the  Soil.  --  The  general 
influence  of  the  forest  is,  on  the  whole,  to  cool  the  soil. 
The  extremes  of  temperature  are  reduced  as  compared 
with  those  of  the  open  fields,  but  the  effect  is  more  marked 
(is  greater)  on  the  summer  maximum  than  on  the  winter 
minimum  temperatures. 

Temperature  of  the  Air. — The  air  temperature  under 
the  crowns  of  forest  trees  is,  on  the  whole,  cooler  —  lower 
in  summer,  and  higher  in  winter  —  than  in  the  open 
fields. 

The  kind  of  trees  composing  a  forest  has  an  influence  on  the  tem- 
perature, although  the  average  for  the  year  is  about  the  same  for  all 
kinds  of  trees.  For  evergreen  trees,  the  difference  in  the  temperatures 
under  the  trees  and  in  the  open  shows  a  symmetrical  increase  and  de- 
crease during  the  year,  being  least  in  winter,  and  greatest  in  summer ; 
but  for  the  deciduous  trees  the  difference  is  variable,  diminishing 
from  midwinter  to  springtime,  but  increasing  rapidly  with  the  growth 
of  leaves.  In  young  forests  the  maximum  air  temperatures  are  lower, 
and  the  minimum  highen,  than  in  old  forests. 

The  air  temperature  within  the  crowns  of  trees  is 
higher  than  for  the  same  elevation  above  ground  in  the 
open  fields. 


300  ELEMENTARY   METEOROLOGY^ 

The  difference  in  temperatures  within  the  crowns  and  below  is  more 
constant  in  evergreen  than  in  deciduous  trees.  There  is  frequently  in 
the  woods,  especially  in  summer  time,  a  higher  temperature  above  than 
below. 

The  air  temperatures  above  the  crowns  of  trees  are  similar  to  those 
over  a  grassy  meadow  or  cornfield,  —  warmer  than  the  air  in  the  sun- 
shine by  day,  and  cooler  by  night.  The  effect  is  greatest  for  evergreen 
trees  and  for  deciduous  trees  in  full  leaf. 


The  temperatures  within  the  tree  trunk  are  a  little 
higher  than  those  of  the  surrounding  air  in  the  early 
spring  and  late  summer,  but  for  the  rest  of  the  year  are 
lower, 

Local  Moisture.  —  The  absolute  humidity  within  a  forest 
slightly  exceeds  that  of  the  open  ;  and  the  relative  humidity 
is  from  2%  to  4  %  greater,  the  excess  being  much  more 
marked  for  evergreen  than  for  deciduous  trees.  It  is  not 
known  that  precipitation  is  greater  over  forests  than  over 
the  open,  but  the  presence  of  trees  retards  the  rapidity 
of  loss  of  fallen  rain. 

In  middle  latitudes  the  annual  evaporation  from  a  sur- 
face within  a  forest  is  about  half  that  in  an  open  field,  and 
the  maximum  evaporation  in  the  forest  occurs  in  May 
or  June. 

Influence  of  Forests  on  the  Surrounding  Climate.  —  The 
water  which  the  roots  of  trees  collect  from  the  ground, 
and  which  passes  through  the  trunk  and  branches,  is  said 
to  be  transpired.  The  water  which  is  evaporated  from  the 
tree  crowns  through  transpiration  is  carried  as  vapor  by 
the  winds  to  the  adjacent  regions,  and  the  moisture  on 
the  leeward  side  of  forests  is  thus  slightly  increased. 

Local  air  currents  arise  due  to  the  difference  between 
the  temperatures  of  the  forest  and  of  the  surrounding 
country.  Cooler  currents  in  the  lower  air  strata  come  from 


CLIMATE.  301 

the  forest  by  day,  and  warmer  ones  in  the  upper  strata  by 
night. 

The  forests  act  as  a  wind  break  for  the  regions  close  by 
on  the  leeward  side,  and  they  also  reduce  the  force  of  the 
wind  in  the  lower  air  layers  by  the  friction  with  the  tree 
tops.  This  last  would  consequently  reduce  the  evapora- 
tion, which  depends  so  much  on  the  wind  velocity. 

When  the  forest  forms  a  glade  around  a  small  open  area 
of  land,  the  climate  of  the  inclosed  space  is  not  so  extreme 
as  it  would  otherwise  be. 

In  dry  regions,  when  the  water  is  stored  underground, 
and  prevented  from  coming  to  the  surface  by  overlying 
hard  strata  of  earth,  then  the  deep  penetrating  roots  of 
forest  trees  bring  this  moisture  to  the  surface,  and  by  tran- 
spiration give  it  to  the  air. 

Altitude  and  Climate.  —  The  climatic  factors  of  a  place 
—  due  to  its  location  as  regards  latitude,  and  continental  or 
oceanic  exposure  —  are  greatly  modified  by  the  altitude  of 
the  place  above  sea  level.  With  increase  of  altitude,  the 
air  pressure,  the  temperature,  and  the  absolute  amount  of 
moisture  in  the  air,  and  above  a  certain  altitude  the  rainfall, 
decrease  ;  while  the  intensity  of  the  direct  solar  rays,  the 
evaporation,  the  winds,  and  up  to  a  certain  altitude  the  rain- 
fall, increase.  Since  these  variations  are  mentioned  under 
the  different  meteorological  elements  in  treating  them 
separately,  they  are  not  dwelt  on  further  here. 

Climatic  Zones.  —  It  has  been  customary  to  divide  the 
climates  of  the  earth's  surface,  according  to  the  solar  cli- 
mate, into  three  zones.  The  tropical  or  warm  zone  lies 
between  the  tropics ;  the  temperate  zone  lies  between  the 
tropic  and  the  polar  circle,  and  extends  from  latitude  23 1° 
to  latitude  66J°  ;  and  the  polar  or  frigid  zone  lies  within 
the  polar  circle,  and  extends  from  latitude  66J°  to  the 


3<D2  ELEMENTARY   METEOROLOGY. 

pole.  The  temperate  and  polar  zones  occur  in  both  the 
northern  and  southern  hemispheres,  and  the  torrid  zone 
extends  from  the  equator  23 J°  into  each  hemisphere;  the 
areas  included  in  these  zones  have  in  each  hemisphere  the 
following  ratio :  torrid  zone  5,  temperate  zone  6.5,  polar 
zone  i. 

Tropical  Climate.  —  The  characteristic  of  the  climate 
of  the  tropical  zone  is  a  uniformity  of  the  climatic  ele- 
ments such  as  exists  in  no  other  zone.  The  periodic 
changes  depending  on  the  daily  and  annual  course  of  the 
sun  are  the  most  pronounced  and  regular,  while  the  un- 
periodic  or  irregular  changes  are  of  but  secondary  impor- 
tance. The  temperature  is  remarkably  constant  throughout 
the  year.  The  rainfall  is  copious.  There  is  a  rainy  and 
slightly  cooler  season,  and  a  dry  and  slightly  warmer  sea- 
son, and  these  are  governed  by  the  character  of  the  wind. 
The  rainy  season  begins  about  the  time  when  the  sun 
reaches  its  greatest  altitude.  The  winds  are  mainly  those 
belonging  to  the  general  atmospheric  circulation  and  the 
monsoons,  and  are  consequently  permanent  throughout  a 
season.  The  weather  has  great  permanency  of  conditions, 
as  the  extensive  cyclones  which  cause  the  frequent  and  great 
weather  changes  in  higher  latitudes  seldom  occur  in  the 
tropics.  During  the  wet  season  or  during  the  dry  season, 
the  weather  of  one  day  is  very  much  like  that  of  another. 

The  relative  and  the  absolute  humidity  are  great,  and  the 
heat  is  consequently  oppressive.  During  the  dry  season 
the  sky  is  usually  clear,  but  during  the  wet  season  gener- 
ally cloudy.  During  the  wet  season,  after  noon,  thunder- 
storms of  great  intensity  occur  with  great  regularity. 

The  twilight  is  short,  and  does  not  vary  much  in  length 
throughout  the  year.  Vegetation  grows  the  year  round 

Temperate  Climate.  —  The  following  are  the  characteris- 


CLIMATE.  303 

tic*  of  the  climate  of  the  temperate  zones  :  The  tempera- 
ture is  subject  to  great  and  sudden  changes  from  day  to 
day;  and  there  is  also  a  great  difference  in  the  temperature 
in  winter  and  in  summer.  During  the  winter  the  tempera- 
ture is  as  low  as,  and  in  some  cases  lower  than,  that  of 
some  regions  of  the  polar  zone ;  while  during  the  summer 
the  temperature  is  in  some  cases  higher  than  that  in  the 
tropical  zone.  The  prevailing  winds  are  from  the  west 
towards  the  east,  but  the  frequent  occurrence  of  extensive 
cyclonic  and  anticyclonic  disturbances  causes  a  great 
variability  of  the  wind,  both  in  direction  and  velocity. 

The  times  and  amounts  of  rainfall  are  very  irregular,  as 
are  also  the  amounts  of  moisture  and  cloud.  These  all 
depend  very  much  on  the  local  conditions  and  the  tem- 
porary character  of  the  wind. 

Vegetation  grows  during  at  least  six  months  of  the  year, 
but  during  the  cold  season  is  at  a  standstill. 

The  climate  of  the  temperate  zone  in  the  southern 
hemisphere  is  much  more  equable  than  in  the  northern 
hemisphere,  on  account  of  the  greater  amount  of  water 
surface  in  the  former.  In  the  temperate  zone  mankind 
has  reached  the  highest  development. 

Polar  Climate.  —  The  principal  characteristic  of  the  cli- 
mate of  the  polar  or  frigid  zone  is  the  absence  of  solar  rays 
during  a  longer  or  shorter  portion  of  the  year.  The  daily 
temperature  change  is  small.  During  the  winter  con- 
stant relatively  low  temperatures  prevail,  and  during  the 
summer  constant  relatively  high  temperatures  are  expe- 
rienced. The  average  temperature  is  lower  than  that  of 
the  temperate  zone,  but  the  actual  minima  are  not  neces- 
sarily lower.  The  winds  are  irregular  in  character,  but 
there  is  greater  frequency  of  calms  than  in  the  temper- 
ate zone.  The  precipitation  is  slight,  and  occurs  mostly 


40  "60  180  160  140  |?0  lOO 


Tropical  Zone 

'Zone. 
Constant  teniperctte. 


Tempt  rate  Zo 


|4Q  160  180  160  140  120  100 


(304) 


FIG.  89.  —  HEAT  ZONES  OF  THE  EARTH,  ACCORDING  TO  r 


60 


DURATION  OF  THE  HOT,  TEMPERATE,  AND  COLD  PERIODS  (AFTER  KOPPEN). 


(305) 


306  ELEMENTARY   METEOROLOGY. 

as  snow.  The  absolute  humidity  is  slight,  but  the  relative 
humidity  and  cloudiness  are  sometimes  excessive.  Only 
special  forms  of  vegetation  grow  at  all,  and  these  during 
but  a  short  season.  It  is  the  least  desirable  climate  for 
the  development  of  mankind. 

The  Heat  Zones  of  the  Earth.  —  It  has  been  found  con- 
venient to  make  a  more  special  division  of  the  earth's  sur- 
face into  seven  heat  zones,  according  to  the  duration  of  the 
hot,  temperate,  and  cold  periods  (Fig.  89,  after  Kb'ppen). 
These  purely  arbitrary  divisions  are  as  follows  :  — 

1.  The  tropical  zone,  which  has  an  average  temperature 
of  over  68°  F.  during  all  months. 

2.  The  subtropical  zone,  which  has  during  from  4  to  1 1 
months  only,  an  average  temperature  of  over  68°  F. 

3.  4,  5.     The  temperate  zone,   which    has  from   4  to    12 
months  when  the  average  temperature  lies  between  50° 
and  68°  F.     It  has  three  subdivisions. 

(3)  The  constant  temperate  zone,  which   has  no  month 
when  the  average   temperature  exceeds  68°   F.,   or  falls 
below  50°  F. 

(4)  The  temperate  zone  with  Jwt  summers,  which   has 
but  few  months  in  which    the  average  temperature  falls 
below  50°  F. 

(5)  The  temperate  zone  with  moderate  summers  and  cold 
winters,  which  has  from  i  to  8  months  with  average  tem- 
perature between  50°  and  68°  F.,  and  from  i  to  8  months 
with  temperatures  below  50°  F. 

6.  The  cold  zone,  which   has  from  i   to  4  months  with 
average   temperature   over    50°.  F.,  while    the    remaining 
months  are  cold. 

7.  The  polar  zone,   which  has,   in   general,  an  average 
temperature  of  less  than  50°  F.  during  all  the  months  of 
the  year. 


CLIMATE.  307 

CLIMATES  OF  THE  CONTINENTS. 

Africa.  —  Africa  is  the  typical  tropical  continent ;  and, 
since  it  extends  from  about  latitude  35°  south  to  about 
the  same  latitude  north,  only  the  northern  and  southern 
extremities  enter  the  temperate  zones,  and  the  main  body 
is  in  the  tropical  zone. 

Central  or  Tropical  Africa.  —  The  climate  of  tropical 
Africa,  which  includes  the  whole  of  the  central  part  of  the 
continent,  is  warm  and  moist,  with  a  dry  and  a  wet  season. 
The  western  side  of  the  continent  is  characterized  by 
great  rainfall  and  relatively  lower  temperature,  while  the 
eastern  coast  is  drier  and  warmer.  The  rainy  season  on 
the  west  coast  ranges  from  August  to  November,  and 
on  the  east  coast  from  November  to  March  or  April. 

Northern  Africa  is  mostly  a  desert,  and  the  air  is  dry 
and  warm.  The  season  of  rain  is  from  October  to  March 
or  April. 

Southern  Africa  has  a  tendency  towards  a  desert  climate 
(as  in  the  north),  except  on  the  eastern  coasts,  where  it  is 
more  nearly  oceanic  in  character.  The  eastern  coast  is 
the  moister  and  cooler. 

Europe.  —  The  climate  of  Europe  is  temperate.  The  pre- 
vailing winds  are  from  the  west,  and  in  western  Europe 
come  from  the  ocean  ;  and  so  the  western  parts,  and  espe- 
cially western  coasts,  are  moist  and  warm,  but  the  eastern 
parts  cool  and  dry. 

In  western  Europe  the  winters  are  mild  and  the  summers 
cool ;  and  the  rainfall  is  well  distributed  through  the  year, 
but  in  most  regions  somewhat  more  rain  falls  in  the  sum- 
mer and  fall  than  at  other  times  of  the  year. 

In  central  and  eastern  Europe  the  winters  are  cold  and 
the  summers  warm.  The  rainfall  is  fairly  well  distributed 


30S  ELEMENTARY   METEOROLOGY. 

through  the  year,  but  is  greatest  in  summer.  The  winters 
grow  colder  and  the  summers  warmer,  and  consequently 
there  are  greater  extremes  of  temperature,  with  the  east- 
ward progress  through  Europe ;  and  the  climate  changes 
from  oceanic  on  the  western  side  to  continental  on  the 
eastern  side. 

Southern  Europe,  which  borders  on  the  Mediterranean 
Sea,  has  a  peculiarly  warm  and  rather  dry  climate,  due  to 
the  warm  winds  from  the  African  desert  and  Mediterra- 
nean Sea  on  the  south,  and  the  protecting  influence  of  the 
Alps  Mountains,  which  keep  out  the  cold  winds  from  the 
north.  The  season  of  greatest  rainfall  is  well  marked,  and 
extends  from  October  to  February. 

Asia.  —  The  climate  of  Asia  is  tropical  at  the  south, 
temperate  at  the  middle  latitudes,  and  polar  in  the  north- 
ern portion. 

Tropical  Asia  is  that  portion  south  of  the  Himalaya 
Mountains,  and  the  extreme  southeastern  part  of  the  con- 
tinent. It  has  a  summer  monsoon  wind  blowing  from  the 
southwest,  and  bringing  the  moist  sea  air  over  the  land,  and 
producing  much  rain,  especially  in  India.  In  winter  the 
monsoon  and  trade  winds  blow  from  the  north  and  north- 
east, and  so  from  the  interior  of  the  continent  over  India, 
and  render  the  air  there  dry  and  cool ;  while  over  the 
extreme  southeastern  part  of  Asia  the  northeast  trade 
winds  bring  neither  such  dry  nor  such  cold  air  as  for 
India  proper.  The  seasons  of  change  of  the  wind  from 
one  monsoon  to  the  other  are  characterized  by  variable 
winds,  hurricanes,  and  stormy  weather.  The  coldest 
weather  is  in  December  or  January,  and  the  warmest 
from  April  to  July.  The  daily  oscillation  of  temperature 
is  excessive  in  the  dry  season,  and  is  small  in  the  rainy 
season ;  but  the  annual  oscillation  is  not  great.  The  hu- 


CLIMATE.  309 

midity  is  great  in  summer,  but  slight  in  winter.  There 
are  three  distinct  seasons,  —  the  cool  season,  from  October 
to  February  or  March ;  the  hot  season,  from  March  to 
July;  and  the  rainy  season,  from  July  to  October.  The 
vegetation  is  active  throughout  the  whole  year,  and  is 
luxuriant. 

At  the  time  of  change  of  monsoons  in  May  and  October, 
cyclones  of  great  energy  sweep  over  the  Bay  of  Bengal, 
and  do  great  damage  at  sea  and  on  the  coast  of  India. 

Temperate  Asia,  which  extends  from  the  Himalaya 
Mountains  and  the  Yangtse  Kiang  northward  to  Siberia, 
is  characterized  by  a  continental  climate.  The  winters  are 
cold  and  dry,  and  the  summers  hot  but  not  very  rainy 
(except  on  the  southern  part  of  the  eastern  coast,  where 
the  rainfall  is  copious).  The  annual  rainfall  of  the  main 
region  north  of  the  Himalayas  is  slight.  Thunderstorms 
are  infrequent.  The  humidity  is  low,  and  the  degree  of 
cloudiness  is  small. 

The  summers  are  warm  enough  to  produce  a  good  vege- 
table growth  where  there  is  sufficient  rainfall. 

The  prevailing  winter  winds  are  from  the  southwest,  but 
in  the  summer  time  northeast  winds  are  as  frequent  as  the 
southwest  winds.  There  are  frequent  calms  in  winter. 

On  the  southern  east  coast  the  precipitation  is  great,  the 
climate  mild,  and  warm  southwest  winds  blow.  In  the 
winter  the  monsoon  winds  are  from  the  northwest.  From 
August  to  October  is  the  season  for  the  terrible  typhoons 
of  this  coast. 

Polar  Asia.  —  The  polar  climate  of  Asia  extends  from 
the  Arctic  Ocean  practically  to  nearly  latitude  50°.  It 
extends  so  much  beyond  the  Arctic  Circle,  on  account  of 
the  marked  continental  features  of  the  climate.  The  win- 
ters are  long  and  intensely  cold ;  the  summers  short  and 


310  ELEMENTARY   METEOROLOGY. 

warm,  except  in  the  extreme  north,  where  the  summer 
heat  is  not  so  great.  The  periodic  daily  temperature 
oscillation  is  large. 

The  absolute  and  the  relative  humidity  are  slight,  and  the 
degree  of  cloudiness  small  for  an  arctic  climate ;  the  win- 
ter sky  being  clear.  The  precipitation  is  slight.  In  the 
winter  the  winds  are  from  the  northwest,  and  in  summer 
from  the  south  and  southeast.  The  short,  hot  summer  is 
favorable  for  the  hardy  vegetable  growth,  and  trees  and 
grains  grow  up  to  a  very  high  latitude. 

On  the  extreme  coast  the  winters  are  milder,  the  sum- 
mers cooler,  and  the  moisture,  cloudiness,  and  rainfall 
increase. 

South  America.  —  The  South  American  continent  ex- 
tends from  10°  north  latitude  to  about  50°  south  latitude. 
It  has  therefore  a  tropical  climate,  except  for  the  southern 
portion,  which  extends  far  into  the  temperate  zone. 

Tropical  South  America  and  Central  America  have  a 
warm,  moist  climate.  The  range  of  temperature  during 
the  year  is  slight  except  just  south  of  the  United  States, 
where  the  cold  north  winds  from  central  North  America 
make  themselves  felt  to  within  15°  of  the  equator.  The 
rainfall  is  excessive  except  on  the  west  coasts,  where  the 
high  Cordilleras  prevent  the  moisture  with  which  the  pre- 
vailing east  winds  are  laden  from  reaching  the  coast.  There 
is,  then,  a  strip  along  most  of  the  west  coast  where  dry  con- 
ditions prevail.  The  greatest  rainfall  occurs  in  general  in 
Brazil  in  the  summer  season  of  the  southern  hemisphere, 
and  during  the  months  of  May  to  August  little  rain  falls ; 
but  the  varieties  of  rainfall  types  throughout  this  whole 
tropical  region  are  numerous. 

Temperate  South  America. — The  temperature  on  the 
east  coast  is  higher  than  on  the  west  coast,  and  inland  it 


CLIMATE.  311 

is  higher  still.  While  the  summer  temperatures  are  high, 
the  winter  temperatures  are  not  nearly  so  low  as  for  a 
similar  latitude  in  North  America,  because  the  winds  from 
the  ocean  temper  the  climate.  On  the  west  coast  the 
annual  variation  of  the  temperature  is  less  than  in  the 
eastern  part  of  the  continent. 

The  rainfall  is  greatest  on  the  eastern  side  in  lower 
latitudes,  where  .the  summer  is  the  rainy  season,  and  on 
the  western  side  in  the  higher  latitudes,  where  the  winter 
is  the  rainy  season.  At  the  southern  point  of  the  con- 
tinent the  rainfall  occurs  about  equally  at  all  seasons.  In 
lower  latitudes  the  winds  on  the  east  side  are  variable,  but 
are  mostly  from  the  north  and  northeast ;  but  on  the  west 
side  south  and  southwest  winds  are  most  frequent.  In 
higher  latitudes  the  prevailing  winds  are  from  the  west. 

On  the  southern  extremity  of  the  continent  the  humidity 
and  the  cloudiness  are  great,  and  the  temperature  equable, 
owing  to  the  oceanic  character  of  the  climate. 

North  America.  —  North  America  has  three  main  types 
of  climate,  —  the  tropical  at  the  south,  the  temperate 
occupying  the  greater  portion  of  the  continent,  and  the 
polar  climate  at  the  north. 

Tropical  North  America  is  characterized  by  hot,  rainy 
summers  and  cool,  dry  winters.  Except  where  high  moun- 
tain ranges  interfere,  the  rainfall  is  copious.  The  cloud- 
iness and  moisture  are  in  excess  in  the  summer,  and 
deficient  in  winter. 

Temperate  and  Arctic  North  America.  — The  climate  of 
the  main  portion  of  North  America  changes  gradually 
from  a  polar  climate  at  the  north  to  tropical  at  the  south. 
There  are  also  three  climatic  zones  to  be  met  with  in  cross- 
ing the  continent  from  the  east  to  the  west. 

The  eastern  zone  extends  from  the  Atlantic  coast  to  the 


312  ELEMENTARY   METEOROLOGY. 

center  of  the  continent,  and  has  hot  summers  and  cold 
winters.  The  humidity  is  great,  and  the  rainfall  is  not  ex- 
cessive, nor  is  it  deficient.  On  the  east  side  of  the  con- 
tinent the  cold  but  not  very  dry  polar  climate  extends 
southward  to  low  latitudes,  and  the  hot,  wet,  tropical  cli- 
mate extends  far  north  into  relatively  high  latitudes,  so 
that  there  are  great  differences  in  temperature  with  slight 
changes  of  latitude.  Frequent  irregular  changes  of  tem- 
perature of  great  magnitude  occur. 

The  central  zone  extends  from  the  center  of  the  con- 
tinent to  the  Pacific  coast  range  of  mountains.  The 
winters  are  cold,  the  summers  hot,  the  moisture  is  de- 
ficient, and  there  is  little  rainfall.  The  irregular  changes 
of  temperature  are  sudden  and  great,  and  the  daily  am- 
plitude of  temperature  oscillation  is  also  great. 

The  western  zone  embraces  a  long  narrow  strip  extend- 
ing from  the  coast  to  the  nearest  high,  mountain  range. 
At  the  center  and  north  the  winters  are  not  cold,  nor  are 
the  summers  very  warm,  although  intense  heat  is  some- 
times experienced ;  and  the  humidity  and  rainfall  are 
excessive  in  winter,  but  deficient  in  summer.  In  the 
extreme  south  the  climate  is  tropical. 

The  prevailing  winds  in  the  eastern  half  of  America  are 
from  the  northwest  in  winter,  and  from  the  southwest  in 
summer.  In  the  western  part  they  vary  greatly  for  differ- 
ent regions  in  winter,  but  are  mainly  from  the  west  and 
south  in  summer. 


CHAPTER    XIII. 
CLIMATE  OF  THE   UNITED  STATES 

(This  chapter  may  be  considered  as  an  appendix.} 

Climatic  Location  of  the  United  States.  —  The  United 
States  lies  almost  wholly  within  the  north  temperate  zone, 
but  extends  somewhat  into  the  subtropical  zone  on  the 
south,  and  into  a  cold  climate  on  the  north.  Since  it  also 
extends  across  the  continent,  and  has  oceans  on  the  east 
and  west,  and  contains  extensive  mountain  ranges,  there 
is  a  great  diversity  of  climate  within  its  boundaries. 

Main  Types  of  Climate  in  the  United  States.  —  There  are 
found  the  three  main  continental  types  of  climate  properly 
belonging  to  middle  latitudes,  where  the  prevailing  winds 
are  from  the  west.  At  the  western  part,  bordering  on  the 
Pacific  Ocean,  there  is  the  relatively  mild  windward  (west) 
coast  climate ;  at  the  east  there  is  a  leeward  (east)  coast 
climate ;  and  at  the  interior  of  the  continent  between  these 
two  regions  is  the  severe  continental  climate.  On  the 
southern  border  the  climate  becomes  almost  tropical  in 
character  in  all  three  of  these  longitudinal  zones. 

The  high  mountain  ranges  lying  nearly  parallel  to  the 
Pacific  coast  and  very  near  to  it  prevent  the  extension  of 
the  western  coast  climate  to  any  great  distance  inland, 
while  the  relatively  low  mountains  in  the  eastern  part  do 
not  prevent  the  interior  continental  climate  from  extending 
close  to  the  eastern  or  Atlantic  coast.  Thus  the  greater 
part  of  the  climate  of  the  United  States  is  that  pertaining 
to  the  interior  of  a  continent  in  middle  latitudes 


ELEMENTARY  METEOROLOGY, 


FIG.  90.  —  PATHS  PURSUED  BY  THE  CENTERS  OF  CYCLONIC  AREAS,  JANUARY,  1893 
(U.S.  WEATHER  BUREAU). 


FIG.  91.  — PATHS  PURSUED  BY  THE  CENTERS  OF  ANTICYCLONIC  AREAS,  JANUARY,  1893 
(U.S.  WEATHER  BUREAU). 


CLIMATE   OF  THE   UNITED    STATES. 


315 


FIG.  92. —  PATHS  PURSUED  BY  THE  CENTERS  OF  CYCLONIC  AREAS,  JULY,  1893 
(U.S.  WEATHER  BUREAU). 


FIG.  93. — PATHS  PURSUED  BY  THE  CENTERS  OF  ANTICYCLONIC  AREAS,  JULY,  1893 
(U.S.  WEATHER  BUREAU). 


316  ELEMENTARY    METEOROLOGY. 

Climatic  Effects  of  Cyclones  and  Anticyclones.  —  There 
is  another  feature  which  much  modifies  the  normal  con- 
ditions of  climate  under  these  circumstances;  and  that  is 
the  relative  frequency,  and  the  location  of  the  paths,  of 
the  great  cyclonic  and  anticyclonic  disturbances  of  middle 
and  higher  latitudes. 

The  region  of  greatest  frequency  of  cyclonic  areas  lies 
in  the  neighborhood  of  the  Great  Lakes,  and  extends  from 
perhaps  the  95th  meridian  through  the  St.  Lawrence 
valley.  To  the  south  and  west  of  this  region  there  is  a 
rapid  decrease  in  their  frequency.  Since  the  passage  of 
all  cyclonic  areas,  and  the  almost  invariably  accompany- 
ing anticyclones,  is  marked  by  the  succession  of  meteoro- 
logical changes  already  described,  the  variability  of  the 
climatic  conditions  increases  in  proportion  to  the  number 
of  cyclones.  We  thus  have  the  most  constant  and  stable 
climate  in  the  regions  least  frequented  by  the  cyclones. 

Cyclones  are  not  only  much  more  frequent,  but  are 
also  much  more  widely  distributed  over  the  country, 
in  the  cold  season  than  in  the  warm.  In  the  latter, 
however,  the  path  of  greatest  frequency  lies  much  far- 
ther north  than  in  the  cold  season.  And  thus,  while  in 
the  winter  the  changes  due  to  the  passage  of  the  cyclones 
frequently  extend  to  the  Gulf  of  Mexico,  yet  in  the 
summer  the  Southern  States  do  not  feel  their  influence  so 
much,  since  they  are  relatively  seldom  visited'  by  these 
atmospheric  disturbances. 

Figs.  90,  91.  92,  and  93  show  the  paths  pursued  by  the  centers  of 
cyclonic  and  anticyclonic  areas  during  the  months  of  January  and 
July,  1893. 

The  arrows  point  in  the  direction  of  motion,  and  the  distances 
between  the  dots  on  the  lines  show  the  movements  of  translation 
during  12-hour  periods. 


CLIMATE  OF  THE   UNITED   STATES  317 

Since  cyclones  and  anticyclones  control  the  weather  to 
distances  of  hundreds  of  miles  from  their  centers,  it  is 
seen  that  in  the  cold  season  almost  the  whole  of  the 
United  States  comes  at  times  within  their  limits  of 
domination.  In  the  warm  season,  although  the  paths  lie 
farther  to  the  north,  yet  the  southern  part  of  the  United 
States  sometimes  comes  within  their  influence. 

The  shifting  of  the  direction  of  the  wind,  and  the  tran- 
sition from  clouded  to  clear  sky,  due  to  the  passage  ot 
these  cyclones  and  anticyclones,  cause  the  sudden  changes 
in  weather  which  produce  the  variability  of  climate  so 
noticeable  in  the  eastern  half  of  the  United  States.  In 
this  latter  portion  of  the  country  there  is  sufficient 
moisture  (brought  by  the  winds  from  the  bodies  of  water 
on  the  north,  east,  and  south)  to  allow  the  full  activity 
of  the  cyclones  in  their  most  complete  forms  to  make 
itself  felt  by  producing  the  characteristics  already  noticed 
for  cyclones ;  especially  those  pertaining  to  the  rapid 
spread  of  the  rain  area  and  copious  rainfall. 

In  the  western  and  drier  half  of  the  United  States, 
however,  the  lack  of  moisture  and  the  breaking-up  of  the 
winds  by  the  mountain  ranges  not  only  prevent  the  full 
development  of  the  characteristics  of  the  cyclonic  areas, 
but  also  render  their  effects  less  marked.  When  the  air 
currents  do  not  supply  much  moisture,  the  growth  of  the 
rain  area  is  not  so  rapid,  and  the  rainfall  is  less  abundant. 

Climatic  Subdivisions  of  the  United  States.  —  The  follow- 
ing systematic  division  of  the  United  States  into  climatic 
subdivisions  was  adopted  by  the  United  States  Weather 
Bureau,  and  is  based  on  the  variations  of  climate  with  lati- 
tude, altitude,  and  marine  exposure  (Fig.  94). 

Starting  with  the  Pacific  coast  and  passing  towards  the 
east,  we  find  six  nearly  parallel  zones  of  quite  distinctive 


ELEMENTARY   METEOROLOGY, 


climatic  character,  stretching  across  the  country  from 
nearly  north  to  south.  These  zones  are  subdivided  lati- 
tudinally  into  northern,  middle,  and  southern  portions. 

i.  The  Pacific  Coast  Region  is  divided  into  the  northern, 
middle,  and  southern  parts,  and  is  the  region  lying  along  the 
Pacific  Ocean,  and  extending  perhaps  200  miles  inland.  It 
has  a  very  equable  temperature  due  to  the  prevailing  west 
winds  from  the  ocean.  The  winters  are  abnormally  warm 
over  the  northern  and  middle  sections.  The  summer  heat, 


FIG.  94.  —  THE  MAIN  CLIMATIC  SUBDIVISIONS  OF  THE  UNITED  STATES  (ADOPTED  BY 
THE  U.S.  WEATHER  BUREAU). 

except  in  the  northern  portion  and  on  the  middle  coast,  is 
sometimes  excessive.  Frosts  seldom  occur.  The  year  is 
divided  into  a  wet  season  in  winter  and  a  dry  season  in 
summer.  The  amount  of  precipitation  is  very  great  at  the 
north,  and  slight  at  the  south,  where  irrigation  is  necessary 
for  the  cultivation  of  crops. 

2o    The  Plateau  Region  extends  longitudinally  from  800 
to  1,000  miles  between  the  Sierras  and  Rocky  Mountains, 


CLIMATE  OF  THE  UNITED   STATES,  319 

and  is  divided  latitudinally  into  the  northern,  middle,  and 
southern  plateau.  It  has  an  altitude  of  several  thousand 
feet,  and  the  air  is  cool  and  dry.  During  the  winter  the 
cold  is  continuous.  In  summer  the  high  altitude  and 
clear  sky  allow  high  temperatures  to  be  reached  in  the 
daytime,  but  at  night  the  radiation  is  rapid5  and  the 
temperature  becomes  low,  so  that  the  daily  range  of  the 
temperature  is  great.  Hot  winds  of  a  foehn  character 
occur  frequently  in  the  valleys.  The  moisture  and  pre- 
cipitation are  deficient  throughout  the  whole  region5  and 
agriculture  must  be  carried  on  mostly  by  irrigation.  The 
high  altitude  causes  the  average  cool  northern  climate 
to  extend  far  into  the  southern  plateau, 

3.  The  Eastern  Slope  of  the  Rocky  Mountains,  or  the 
Great  Plains,  extends  about  500  miles  longitudinally  from 
the  Rocky  Mountains  to  the  low  prairie  lands  of  about  a 
thousand  feet  altitude,  and  is  divided  into  the  northern, 
middle,  and  southern  slopes.  It  has  a  truly  continental 
climate,  which  is  characterized  by  cold  winters  and  hot 
summers.  Great  and  sudden  changes  of  temperature 
occur,  In  winter,  extreme  cold  occurs  on  the  northern 
slope,  and  this  cold  air  is  often  carried  far  into  the  south- 
ern slope  (Texas)  by  the  north  wind  at  the  rear  of  cyclones. 
The  precipitation  is  slight  at  the  west,  where  irrigation  is 
necessary  for  agriculture,  but  increases  towards  the  east. 
The  greatest  rainfall  is  in  early  summer  on  the  northern 
slope,  and  in  late  summer  on  the  southern  slope.  The 
average  wind  velocities  over  the  whole  slope  are  excessive, 
mainly  owing  to  the  lack  of  obstructions  to  the  wind,  such 
as  mountains  and  forests. 

4o  The  Central  Prairie  Lands  extend  to  the  eastward 
of  the  eastern  slope  longitudinally  400  or  500  miles  to  the 
Mississippi  River,  and  are  divided  as  follows  :  the  Missouri 


32O  ELEMENTARY   METEOROLOGY. 

valley,  the  Upper  Mississippi  valley  (the  northern  part,  the 
Red  River  and  Upper  Missouri  valleys,  is  sometimes  called 
the  Extreme  Northwest,  or  simply  Dakota),  and  the  Western 
Gulf  Region  lying  to  the  west  of  the  Lower  Mississippi. 
These  have  a  continental  climate  with  cold  winters  and 
hot  summers  in  the  Extreme  Northwest,  the  Missouri, 
and  Upper  Mississippi  river  valleys ;  but  in  the  Western 
Gulf  Region  the  Gulf  of  Mexico  exerts  a  modifying  influ- 
ence when  the  wind  is  from  the  south,  while  the  "  northers  " 
bring  the  cold  from  the  north  into  this  region  in  winter. 
The  rainfall  is  deficient  at  the  north,  but  increases  to  a 
normal  amount  (about  40  inches)  towards  the  center, 
and  becomes  excessive  near  the  Gulf  of  Mexico.  The 
winds  are  strong,  and  usually  from  the  west  in  the  north- 
ern part ;  but  in  the  Western  Gulf  Region  they  are  weaker, 
and  move  from  the  east  and  south.  The  Rio  Grande  val* 
ley  has  a  climate  partaking  of  some  of  the  characteristics 
of  both  the  Western  Gulf  Region  and  the  Southern  Slope. 

5.  The  Western  Appalachian  Slope  extends  to  the  Mis- 
sissippi River,  and  includes  the  Upper  and  Lower  Lake 
Region  at  the  north,  the  Ohio  and  Tennessee  valleys  at  the 
center,  and  the  Eastern  Gulf  Region  at  the  south.  In  the 
Lake  Region  there  is  a  cold  winter,  but  not  a  very  hot 
summer,  the  extremes  being  tempered  by  the  large  bodies 
of  water  The  winds  are  strong,  and  mostly  from  the 
west. 

In  the  Ohio  and  Tennessee  river  valleys  the  summers 
are  hot,  and  the  winters  short  and  of  but  moderate  cold- 
ness. The  winds  are  not  excessive,  and  blow  mostly 
from  the  west  and  southwest. 

In  the  Eastern  Gulf  States  the  winters  are  very  short, 
and  ice  seldom  forms.  The  summers  are  very  long  and 
hot ;  but  the  great  amount  of  forest,  and  the  winds  trom 


CLIMATE   OF  THE  UNITED   STATES.  321 

the  Gulf  of  Mexico;  somewhat  temper  the  heat.  The  winds 
are  from  northern  directions  in  winter,  and  from  southern 
directions  in  summer. 

The  rainfall  is  about  normal,  and  most  frequent  in  the 
early  summer,  in  the  Lake  Regions  and  Ohio  and  Tennes- 
see valleys ;  but  it  is  excessive,  and  most  frequent  in  late 
summer  and  early  fall,  in  the  Eastern  Gulf  States. 

60  The  Atlantic  Slope  has  at  the  north  the  New  England 
region,  at  the  center  the  Middle  Atlantic  States  as  far 
south  as  North  Carolina,  and  at  the  south  the  South 
Atlantic  States,  except  the  middle  and  southern  parts  of 
Florida. 

The  climate  of  the  Atlantic  coast  does  not  differ  much 
from  that  of  the  western  Appalachian  slope  at  the  same 
latitudes.  The  winters  are  quite  severe  on  account  of  the 
prevailing  west  and  northwest  winds,  which  blow  the  cold 
air  from  the  inland  towards  the  coast.  The  summers  are 
warm  because  the  winds  are  from  the  west  and  southwest, 
and  these  blow  the  hot  continental  air  coastward.  The 
prevailing  winds  being  towards  the  ocean  (except  at  the 
extreme  south),  the  moderating  influence  of  the  oceanic  air 
is  little  felt.  The  rainfall,  which  is  well  distributed  through 
the  year,  increases  from  the  north  towards  the  south. 

Central  and  southern  Florida  has  an  insular,  almost 
tropical  climate. 

GEOGRAPHICAL    DISTRIBUTION    OF   THE   CLIMATOLOGICAL 
ELEMENTS  OVER  THE  UNITED  STATES. 

This  is  best  shown  by  means  of  charts,  giving  the  distri- 
bution of  the  climatic  elements.  The  charts  are  supple- 
mented by  a  few  remarks  concerning  the  main  features  of 
this  distribution. 


322  ELEMENTARY   METEOROLOGY, 

Temperature, 

Temperatures  of  the  United  States.  —  Outside  of  the 
tropics,  the  average  temperature  for  the  year  does  not  fully 
represent  the  climate  of  a  place  or  region,  because  the 
winters  are  cold,  and  the  summers  are  warm,  and  the  aver- 
age annual  temperature  does  not  show  this  oscillation.  But 
the  average  temperature  for  the  coldest  month,  and  also  for 
the  warmest  month,  does  show  the  annual  extremes  of 
seasonal  temperature,  and  the  average  of  the  two  is  very 
nearly  the  average  temperature  for  the  year. 

Since  the  United  States  lies  mainly  in  a  single  solar  zone 
(the  north  temperate),  the  coldest  and  the  warmest  months 
are  practically  the  same  over  the  whole  country.  We  shall 
therefore  take  the  temperatures  for  the  midwinter  month 
of  January,  and  the  midsummer  month  of  July,  as  indica- 
tive of  the  extreme  average  seasonal  temperatures.  In 
addition  will  be  given  the  maximum  and  minimum  tem- 
peratures, the  period  (number  of  days)  during  which  the 
temperature  remains  below  the  freezing  point  of  water,  the 
variability  of  average  changes  of  temperature  from  one 
day  to  the  next,  and  the  times  of  earliest  and  latest  frost. 

The  January  Average  Temperatures  in  the  United  States 
are  shown  on  the  accompanying  chart  (Fig,  95).  The 
coldest  region,  with  a  temperature  of  5°  F.  below  zero,  is 
found  near  the  center  of  the  continent,  where  the  outward 
radiation  is  greatest  and  the  warming  influence  of  the 
ocean  is  least  felt,  and  lies  in  North  Dakota  and  Minne- 
sota, and  extends  up  into  British  America.  From  this 
region  the  temperatures  increase  towards  the  south,  west, 
and  east.  The  natural  southward  increase  with  decrease 
of  latitude  is  quite  regular,  and  continues  to  the  Gulf  of 
Mexico,  where  a  temperature  of  55°  F.  is  found;  but  in 


CLIMATE  OF  THE  UNITED   STATES. 


323 


a  southeast  direction  it  reaches  70°  F.,  at  Key  West,  Fla. 
To  the  east  of  the  central  cold  region,  since  the  prevailing 
westerly  winds  carry  the  interior  continental  cold  almost 
to  the  coast,  the  increase  is  gradual,  to  the  temperature 
of  5°  Fo  or  10°  F.  on  the  Atlantic  coast  in  the  same 
latitude.  To  the  west,  however,  the  increase  is  quite 
rapid,  to  40°  F.  on  the  Pacific  coast.  This  rapidity  of 


FIG.  95       AVERAGE  TEMPERATURE  FOR  JANUARY  IN  THE  UNITED  STATES  (AFTER  GREELY). 

change  is  due  to  the  abnormal  warming  of  the  western 
coast  by  the  ocean  wind  from  the  west.  Towards  the 
southwest  the  increase  is  at  first  rapid,  and  then  slow 
through  Utah  and  Colorado,  owing  to  the  altitude,  and 
then  rapid  to  about  55°  F,  on  the  Pacific  coast. 

Along  the  Pacific  coast  there  is  a  decrease  of  temperature  from  55°  Fo 
at  latitude  33°  north,  to  40°  F,  at  latitude  48°  north ;  while  on  the 
Atlantic  coast  the  temperature  at  latitude  25°  north  is  70°  F.,  at  33° 
north  is  50°  F.,  at  45°  (latitude  of  central  Maine)  is  20°  F.,  and  at 
latitude  48°  north  is  probably  about  5°  F.  This  difference  between  the 
two  coasts  is  because  of  the  prevailing  winds  from  the  west,  which  give 


324 


ELEMENTARY   METEOROLOGY, 


to  the  west  coast  more  of  the  character  of  an  oceanic  climate.  The 
Gulf  coast  region  to  the  west  of  the  Mississippi  River  is  considerably 
colder  than  that  to  the  east,  the  difference  reaching  even  10°  F.  at  the 
same  latitude  in  the  extreme  parts  of  these  regions,  because  the  abnor- 
mal cold  at  the  center  of  the  continent  influence^  the  coast  region  very 
strongly  by  means  of  the  cold  winds  from  the  north. 

The  coldest  month  is  January,  but  the  coldest  days  may  occur  in  any 
other  winter  month. 


FIG.   96. — AVERAGE  TEMPERATURE  FOR  JULY  IN  THE  UNITED  STATES  (AFTER  GREELY). 

The  July  Average  ^Temperatures  (Fig.  96).  —  In  this 
midsummer  month  the  average  temperature  is  about  65°  F. 
along  nearly  the  whole  northern  boundary  of  the  United 
States.  There  is  a  gradual  increase  with  southward  prog- 
ress from  this  northern  limit,  to  about  82°  F.  on  the 
Atlantic  and  eastern  Gulf  coasts,  to  85°  F.  on  the  west- 
ern Gulf  coast,  and  to  over  90°  F.  at  the  base  of  the 
southern  plateau  (in  southern  Arizona  and  southeastern 
California).  Along  nearly  the  whole  Pacific  coast  there  is 
a  narrow  belt  having  a  temperature  of  but  60°  F.,  due  to 
the  winds  from  the  relatively  cool  Pacific  Ocean. 


CLIMATE  OF  THE   UNITED    STATES.  325 

On  this  chart  we  see  the  effects  of  the  abnormal  heat- 
ing of  the  land  surface  in  summer.  The  coasts  are  not 
now  warmer  than  the  interior  for  the  same  latitude ;  and 
the  isotherms  bend  poleward  with  removal  from  the  coasts. 

The  isotherms  on  the  Pacific  slope  lie  farther  apart  in 
January  than  in  July,  which  indicates  a  much  more 
gradual  change  of  temperature  for  a  given  distance  in 
winter  than  in  summer.  In  the  rest  of  the  country  the 
isotherms  lie  farther  apart  in  July  than  in  January,  and 
the  change  of  temperature  is  more  gradual  in  summer 
than  in  winter.  Note  the  effect  of  altitude  in  the  Rocky 
Mountain  region  in  carrying  the  summer  isotherms  south- 
ward. 

The  warmest  month,  but  not  necessarily  that  with  the 
hottest  days,  is  July;  but  there  are  some  exceptions,  as, 
for  instance,  on  the  Pacific  coast  August  has  the  highest 
average  temperature. 

Extremes  of  Temperature The  absolute  maximum 

temperatures  observed  in  the  shade  in  the  United  States 
are  shown  on  the  chart,  Fig.  97.  The  variations  in  the 
extreme  maximum  temperatures  over  most  of  the  inland 
are  surprisingly  slight.  It  is  seen,  however,  that  in  the 
region  which  is  deficient  in  moisture,  and  where  the  sum- 
mer sky  is  almost  cloudless,  the  maximum  temperatures 
rise  5°  or  10°,  or  even  in  the  very  dry  deserts  20°,  above 
the  average  maximum  temperatures  for  the  whole  coun- 
try, which  may  be  put  at  a  little  over  100°  F.  Along 
the  ocean  coasts  the  modifying  influence  of  the  cooler  sea 
reduces  the  average  maximum  temperature  for  the  whole 
country  by  about  10°  ;  so  that  the  excess  caused  by  desert 
conditions  about  equals  in  magnitude  the  deficiencies  due 
to  the  oceanic  influence.  The  climatic  influence  of  the 
prevailing  winds  is  not  markedly  felt  on  the  maximum 


326  ELEMENTARY   METEOROLOGY. 

temperature  except  very  near  the  coast,  and  even  there 
the  diurnal  land  and  sea  breezes  play  the  important  role. 
In  most  of  the  eastern  half  of  the  United  States  100°  F. 
is  reached  except  on  the  New  England  coast  and  in  the 
Appalachian  Mountains.  To  the  west  of  about  the  95th 
meridian,  temperatures  of  105°  F.  are  reached,  except  on 
the  Middle  and  Southern  Plateau,  where  they  are  not  over 


FIG.  97.  —  ABSOLUTE  MAXIMUM  SHADE  TEMPERATURE  IN  THE  UNITED  STATES 
(AFTER  GREELY). 

100°  F.  In  central  California  temperatures  of  1 10°  F., 
and  in  southeastern  California  122°  F.  (or  higher)  and 
about  the  same  in  southwestern .  Arizona,  are  reached. 
On  the  Pacific  coast  they  vary  from  90°  F.  at  the  north 
to  100°  F.  at  the  south.  These  high  temperatures  are 
much  more  easily  borne  in  the  dry  air  of  the  west  than 
are  lower  temperatures  in  the  moist  air  of  the  eastern 
United  States. 

The  Minimum  Temperatures  observed  in  the  United  States 
have  much  more  sharply  defined  limits  than  the  maximum 


CLIMATE  OF  THE   UNITED   STATES. 


327 


temperatures  just  mentioned,  and  are  shown  in  Fig.  98. 
The  relative  general  distribution  of  the  minimum  tempera- 
tures is  much  the  same  as  that  for  the  January  average 
temperatures,  shown  on  Fig.  95. 

At  the  center  of  the  continent,  where  the  radiation  of 
heat  is  strongest,  there  is  a  region  of  lowest  minimum 
temperatures,  but  there  is  a  moderating  of  these  extreme 


FIG.  98.  —  ABSOLUTE  MINIMUM  SHADE  TEMPERATURE  IN  THE  UNITED  STATES 
(AFTER  GREELY). 

temperatures  toward  the  east,  west,  and  south  ;  i.e.,  toward 
the  coasts  and  the  equator.  The  general  direction  of  the 
winds  influences  this  distribution  most  strongly.  Blowing 
as  they  do  from  the  west,  they  make  the  temperatures  on 
the  Pacific  coast,  where  the  winds  blow  from  the  ocean, 
from  20°  to  30°  higher  than  on  the  Atlantic  coast,  where 
the  cold  air  is  blown  from  the  center  of  the  continent  east- 
ward. These  extreme  minimum  temperatures  in  the  cen- 
tral and  eastern  United  States  occur  in  the  extended  cold 
waves  which  sweep  over  the  land.  The  great  extent  and 


;V!,X  ELEMENTARY    MKTE<  )K<  >I.<)<  IV 

regular  progressive  motion  of  these  cold   waves  cause  the 
existing  regularity  in  the  line  of  equal   minimum   tempera 
ture  in  the  central  and  eastern  United  Stales. 

The  extreme  minimum  of  —63°  F.  obseived  in  northern 
Montana  (Fort  Assinihoine)  is  surrounded  hy  higher 
temperatures  to  the  east,  west,  and  south.  Towards  the 
west  from  Montana  theie  is  at  first  a  «;radual  and  then 
a  moie  rapid  increase  too"  F.  near  the  coast,  and  on  the 
North  Pacific  coast  itself  the  minimum  is  from  -|  X1  to 
}  15  I1'-  above  zero,  Towards  the  east  from  Montana 
there  is  a  gradual  increase  to  25°  F.  or  —30°  F.  on  the 
Atlantic  coast  in  the  same  hi-h  latitude.  Towards  the 
south  !mm  Mont, in, i  there  is  an  Increase  to  4-13°  I1',  ill 
southern  Texai;  towards  the  southeast  there  is  an  increase 
to  +41°  I'",  at  the  southern  end  of  Florida;  and  towards 
the  .soul  Invest  their  is  an  inciease  to  +32"  F.  on  the 
southwestern  coast  of  (  'alifoi  nia. 

On  the  summit  of  Mount  Washington  in  New  Hamp- 
shire the  minimum  temperature  leached  is  50"  F.  at  an 
altitude  of  about  a  mile',  and  on  Pike's  Peak,  Col.,  about 
—  40°  F.  at  an  altitude  of  about  two  miles  and  a  half 

The  Absolute  Oscillation  of  Temperature  (Max. -M in.)  is 

shown  in   Fi<;.  <)<>. 

Thisehait  brings  out  verv  plainly  the-  fact  that  the  am- 
plitude of  t  he  tempera!  me  depends  to  a  \  it  extent 
on  the  minimum  temperatures  rcaehed,  and  is  but  slightly 
influenced  hy  the  maximum  temperatures  reached.  The 
amplitud'  it  the  interior  <>f  the  continent, 
but  decrease  with  approach  towards  the  coast  and  with 
the  latitude  ,  although  this  latter  decrease  mav  be  almost 
masked  on  a  windward  CO  n  lor  s« »  i  breadth 
of  latitude  as  the  width  of  the  United  Stales.  On  the 

leeward  (eastern)  coast    «»t    the  I'uited   states   the  conti 


CLIMATE  OF  THE   UNITED    STATES. 


329 


nental  influence  is  shown  by  the  decrease  with  latitude 
coming  halfway  between  that  for  the  interior  of  the  conti- 
nent and  the  windward  (Pacific)  coast. 


FIG.  99.  — ABSOLUTE  AMPLITUDE  OF  OSCILLATION  OF  SHADE  TEMPERATURE  IN  THE  UMJED 
STATES  (AFTER  GREELY). 

The  Duration  of  Temperatures  below  Freezing  in  the 
United  States  is  shown  by  the  following  chart  (Fig.  100), 
on  which  is  given  the  number  of  days  when  the  average 
daily  temperature  was  below  32°  F  The  most  southern 
line  drawn  on  the  map,  and  marked  o,  is  the  dividing 
line  between  the  regions  where  the  average  daily  tem- 
perature goes  below  freezing,  and  those  where  it  does  not. 
The  next  line,  marked  30,  passes  through  the  localities 
which  have  30  days  with  the  average  temperature  below 
freezing.  The  additional  lines  are  similarly  drawn  for  the 
successive  multiples  of  30  days  (i.e.,  months). 

This  distribution  follows  in  a  general  way  that  of  the 
midwinter  average  (Fig.  95)  and  minimum  (Fig.  98)  tem- 
peratures already  described.  In  northern  Minnesota  the 


330 


ELEMENTARY  METEOROLOGY. 


number  of  days  is  165,  and  there  is  a  decrease  from  there 
in  all  directions  in  the  United  States.  The  decrease  is 
very  gradual  to  about  1 20  days  on  the  northern  New  Eng- 
land coast  on  the  east,  but  more  rapid  to  o  days  on  the 
Delaware  coast  on  the  southeast,  central  Texas  on  the 
south,  central  Arizona  on  the  southwest,  and  central 
Oregon  on  the  west. 


FIG.    ioo.  —  NUMBER  OF  DAYS  WITH  AVERAGE  TEMPERATURE  BELOW   FREEZING  IN  THE 
UNITED  STATES  (AFTER  GREELY). 

Variability  of  Temperature  in  the  United  States.  —  The 
change  in  the  average  temperature  from  one  day  to  the 
next  varies  greatly  in  the  United  States,  not  only  dur- 
ing short  intervals  of  time,  but  also  on  the  average  for 
each  month  from  month  to  month.  Such  changes  are 
much  greater  in  some  regions  than  in  others. 

The  variability  of  the  temperature  depends  largely  on 
the  descent  of  the  minimum  temperatures,  and  follows 
quite  closely  the  distribution  of  the  minimum  tempera- 
tures (Fig.  98).  The  variability  is  greatest  in  the  interior 


CLIMATE  OF  THE   UNITED   STATES. 


331 


of  the  continent,  but  decreases  towards  the  coasts,  and 
towards  the  windward  (western)  coast  much  more  rapidly 
than  towards  the  leeward  (eastern)  coast.  There  is  also 
a  decrease  towards  the  equator,  scarcely  perceptible  on 
the  windward  coast,  very  marked  at  the  interior,  and 
between  the  two  on  the  leeward  coast. 

In  general,  the  variability  is  greatest  in  midwinter  (Jan- 


FIG.  101.  —  VARIABILITY  OF  AVERAGE  DAILY  TEMPERATURE  IN  JANUARY  IN  THE  UNITED 
STATES  (AFTER  GREELY). 

uary  and  February),  and  least  in  midsummer  (July  and 
August).  The  frequency  and  magnitude  of  these  changes 
are  mainly  dependent  on  the  number,  rapidity  of  move- 
ment, and  intensity  of  the  areas  of  high  and  low  baro- 
metric pressure  which  sweep  over  the  country.  The 
accompanying  chart  (Fig.  101)  shows  the  average  tem- 
perature variability  for  the  maximum  month  of  January. 
It  is  seen  that  the  greatest  variability  occurs  in  the  cen- 
tral interior  region  of  the  northern  part  of  the  United 
States,  where  it  is  about  10.5°  F.  Towards  the  west  of 


WALDO    METEOR.  —  2O 


332  ELEMENTARY   METEOROLOGY. 

this  region  of  maximum  change,  the  variability  decreases 
at  first  slowly,  and  then  more  rapidly  $  to  only  2.5°  F. 
on  the  northern  Pacific  coast.  Towards  the  southwest 
the  decrease  is  to  2°  F.,  on  the  southern  Pacific  coast. 
Towards  the  south  the  decrease  is  to  6°  F.  on  the  Gulf 
coast ;  towards  the  southeast,  to  3°  in  southern  Florida ; 
while  towards  the  east  there  is  but  a  slight,  if  any,  de- 
crease to  the  North  Atlantic  coast. 

The  increase  of  the  average  temperature  variability  with 
latitude  is  scarcely  perceptible  on  the  Pacific  coast  and  in 
the  extreme  western  United  States  in  general;  it  is  about 
i°  for  each  300  miles  at  the  center  of  the  continent;  and 
on  the  Atlantic  coast  the  increase  is  quite  rapid  on  the 
northern  and  southern  coasts,  but  very  gradual  on  the 
middle  coast. 

Relative  Frequency  of  Cold  Waves.  —  The  cold  waves 
that  sweep  over  the  country  in  the  rear  of  cyclonic  disturb- 
ances are  an  important  feature  in  the  climate  of  the  United 
States,  and  the  number  of  rapid  changes  in  temperature 
depends  on  their  frequency.  The  amount  of  these  sudden 
falls  of  temperature  is  quite  a  different  matter  from  the  aver- 
age variability  of  the  temperature  just  mentioned,  and  bears 
somewhat  the  same  relation  to  it  as  the  absolute  range 
of  temperature  bears  to  the  average  range  of  temperature. 
It  is  not  the  coldest  region,  however,  that  has  the  greatest 
number  of  rapid  changes  in  the  temperature.  It  is 
rather  where  the  cold  of  the  center  of  the  continent 
struggles  to  render  abnormally  cold  the  air  of  the  region 
in  the  neighborhood  of  the  ocean  or  other  large  bodies 
of  water,  and  where  the  cyclonic  areas  occur  most  fre- 
quently. Thus  there  arises  in  that  region  the  alternat- 
ing control  of  the  continental  cold  and  the  oceanic  warmth. 
The  accompanying  chart  (Fig.  102)  shows  roughly  the  rela- 


CLIMATE   OF  THE   UNITED   STATES. 


333 


tive  frequency,  in  the  central  and  eastern  United  States,  of 
falls  of  temperature  of  at  least  20°  F.  in  24  hours.  Forty 
such  temperature  changes  occur  in  the  region  of  northern 
Michigan  for  every  five  in  the  region  of  the  southeastern 
United  States. 

Dates  of  Earliest  and  Latest  Frosts. — The  times  of 
earliest  frost  in  the  fall  and  latest  frost  in  the  spring  are 
of  great  practical  importance. 

Average  Date  of  Earliest  Killing  Frost. — This  phe- 
nomenon is  dependent  both  on  the  average  temperature, 


FIG.  102.  —  RELATIVE  FREQUENCY  OF  FALLS  OF  TEMPEKATURE  OF  OVER  20°  IN  24  HOURS 
(AFTER  RUSSELL). 

consequently  on  the  varying  altitude  of  the  sun,  and  on 
the  diurnal  range  of  temperature,  and  consequently  on  the 
cloudiness  and  distance  from  oceans.  Whenever  the  aver- 
age temperature  in  its  autumnal  decrease  reaches  the  place 
where  the  diurnal  range  will  take  the  night  minimum  tem- 
perature below  freezing,  then  the  first  frost  must  occur  ;  but 
this  regular  occurrence  is  usually  anticipated  by  the  ap- 


334 


ELEMENTARY   METEOROLOGY. 


pearance  of  one  of  the  coid  waves  just  mentioned,  in  which 
the  temperature  fall  is  far  greater  than  would  occur  during 
the  regular  diurnal  change.  Frost  occurs  earliest  (Fig.  103) 
at  the  Canadian  boundary  ;  but  the  time  is  retarded,  at  first 
gradually,  and  then  more  rapidly,  with  southern  progress  to 
the  Gulf  coast.  There  is  also  a  retardation  from  Montana 
towards  the  northwest  coast  of  the  United  States.  Frost 
occurs  in  the  northern  part  of  the  United  States  about 
Sept.  i  ;  but  the  time  is  gradually  retarded  to  Oct.  I  at 


FIG.  103.  —  AVEKAGE  DATE  OF  EARLIEST  HARD  FROST  IN  THE  UNITED  STATES 
(AFTER  GREELY). 

about  the  latitude  of  the  Ohio  River  valley,  and  to  Nov.  i 
in  central  Mississippi ;  to  Dec.  i  in  southern  Louisiana,  and 
nearly  to  Jan.  I  in  central  Florida  (when  it  occurs  at  all  in 
the  last-named  region). 

Average  Date  of  Latest  Killing  Frost, —  When  with  the 
increase  of  the  average  temperatures  in  the  spring  (with 
the  altitude  of  the  sun)  these  become  so  high  that  the 
diurnal  change  is  such  as  to  keep  the  night  minimum 


CLIMATE   OF  THE   UNITED   STATES. 


335 


temperature  above  the  freezing  point,  then  ordinarily 
the  frosts  will  cease ;  but  some  temperature  falls  greater 
than  that  due  to  the  diurnal  change  will  very  likely 
occur  at  a  later  date,  and  thus  cause  a  considerable  retarda- 
tion in  the  time  of  latest  frost.  The  latest  frost  occurs 
(Fig.  104)  earliest  along  the  coast  of  the  Gulf  of  Mexico; 
but  the  time  is  retarded,  at  first  rapidly,  but  afterwards 
more  slowly,  to  the  Canadian  boundary.  It  occurs  about 
Feb.  i  on  the  Gulf  coast,  March  I  in  southern  Mississippi, 


FIG.  104.  —  AVERAGE  DATE  OF  LATEST  HARD  FROST  IN  THE  UNITED  STATES 
(AFTER  GREELY). 

April   i   in  northern  Tennessee,  May   I   in  southern  Wis- 
consin, and  the  latter  part  of  May  in  North  Dakota. 

Rainfall 

The  rainfall  conditions  of  the  United  States  as  a  whole 
are  very  variable,  the  total  annual  amount  of  precipitation 
varying  from  3  or  4  inches,  or  less,  to  over  100  inches. 

The  accompanying  chart  (Fig.   105)  shows  the  average 


(336) 


CLIMATE  OF  THE  UNITED   STATES.  337 

annual  precipitation  for  the  different  regions  of  the  whole 
country. 

The  Regions  of  Most  Rainfall  are:  the  northwest  coast, 
where  in  the  extreme  corner  over  100  inches  fall,  owing  tr 
the  warm  moist  air  from  the  Pacific  Ocean  blowing  on 
to  the  cooler  continent ;  the  northeastern  shore  of  the 
Gulf  of  Mexico,  where  over  60  inches  fall,  and  where  a 
plentiful  supply  of  moisture  is  found  in  the  southerly  winds 
from  the  Gulf;  the  southeast  coast  'of  Florida*  and  the 
extreme  eastern  portion  of  the  North  Carolina  coast,  where 
over  70  inches  fall,  and  where  a  plentiful  amount  of  mois, 
ture  is  furnished  by  the  easterly  winds  blowing  from  the 
Gulf  Stream  in  the  Atlantic  Ocean, 

The  Region  of  Least  Rainfall  is  in  the  southwestern 
part  of  Arizona,  where  the  winter  winds  blow  from  the  dry 
interior  of  the  continent,  and  the  summer  winds  blow  from 
a  cooler  to  a  hot  region,  and  thus  little  rain  falls. 

It  is  seen  from  the  map  that  there  is  an  increase  of 
rainfall  with  northward  progress  on  the  Pacific  coast,  and 
a  decrease  from  south  to  north  on  the  Atlantic  coast.  In 
the  eastern  inland  part  of  the  United  States  there  is  a 
decrease  from  south  towards  the  north  ;  in  the  central  part, 
in  the  neighborhood  of  the  looth  meridian,  there  is  no 
change  in  a  north-and-south  direction,  but  rapid  changes 
in  an  east-and-west  direction  ;  while  in  the  extreme  west- 
ern inland  region  there  is  an. increase  with  northward  prog- 
ress. The  region  separating  the  western  from  the  eastern 
type  of  variation  with  latitude  is  almost  at  the  center  of 
the  continent. 

The  wind  from  the  west  and  southwest  blows  the  moist 
Pacific  Ocean  air  over  the  land  ;  and  where  the  land  is 
cooler,  as  on  the  northern  Pacific  coast,  an  excessive  rain- 
fall occurs,  but  where  the  land  is  warmer,  as  on  the  south- 


338  ELEMENTARY  METEOROLOGY 

ern  coast,  but  slight  precipitation  occurs,  Moreover,  the 
sea  wind  at  the  south  is  frequently  but  a  day  wind,  and  is 
not  as  permanent  as  in  the  north. 

The  air  flowing  into  the  northwestern  United  States  is 
deprived  of  most  of  its  moisture  before  it  has  gone  far 
inland,  so  that  the  regions  to  the  east  have  dry  air  and 
little  precipitation  until  moist  air  is  supplied  by  winds 
from  the  Hudson  Bay,  the  Great  Lakes,  the  Atlantic 
Ocean,  or  the  Gulf  of  Mexico.  And  the  northwest  and 
northern  winds  carry  this  dry  air  far  to  the  south,  .almost 
to  the  Gulf  of  Mexico,  making  the  whole  region  traversed 
one  of  deficient  rainfall.  But  from  the  region  where  the 
southwest  winds  begin  to  carry  the  moist  air  from  the  Gulf 
of  Mexico  up  into  the  Mississippi  Valley,  the  rainfall  in- 
creases again  rapidly  over  the  whole  eastern  United  States0 
The  excessive  precipitation  of  the  southeastern  United 
States  is  due  to  the  prevalence  of  warm  oceanic  winds. 

The  whole  United  States  is  divided  into  three  great  distinct  regions 
as  regards  rainfall. 

The  first  region,  to  the  east  of  the  97th  meridian  (which  passes 
through  central  Kansas),  has  an  average  rainfall  of  over  35  inches  ;  and 
while  there  is  a  decrease  from  nearly  60  to  35  inches  with  northward 
progress,  yet,  on  the  whole,  the  distribution  of  the  amount  of  rainfall  is 
remarkably  equable  for  such  an  extended  region. 

The  second  region,  that  to  the  west  of  the  97th  meridian  (central 
Kansas),  has  at  first,  for  the  next  5°  of  longitude,  a  rapid  decrease 
from  35  to  15  inches  of  rainfall  at  the  western  border  of  Kansas; 
but  from  this  (iO2d  meridian)  westward  there  is  little  change  in  the 
rainfall  north  of  latitude  42°  (the  southern  boundary  of  Wyoming)  to  the 
region  of  the  northwest  coast,  where  the  increase  becomes  very  rapid 
towards  the  coast.  £rom  about  the  looth  meridian  westward,  and 
south  of  latitude  40°,  there  is  a  slow,  somewhat  variable  decrease  from 
20  inches  to  a  minimum  rainfall  of  from  2  to  5  inches  in  the  desert 
region  to  the  east  of  the  Sierra  Nevada  Mountains,  from  which  there  is 
a  slight  increase  to  the  Pacific  coast.  The  rapid  change  in  rainfall  in 


CLIMATE  OF  THE   UNITED    STATES 


339 


the  region  just  east  of  the  Rocky  Mountains  is  due  to  the  fact  that  just 
there  the  moisture  begins  to  be  received  from  the  Gulf  of  Mexico  on 
the  south,  and  Hudson  Bay  and  the  Great  Lakes  on  the  north. 

The  third  region  is  on  the  northern  half  of  the  Pacific  slope,  to  the 
west  of  the  Cascade  Mountains.  Here  the  rainfall  increases  rapidly 
(from  25  inches)  from  northern  California  to  the  northern  boundary 
of  Washington,  and  from  the  Cascade  Mountains  on  the  east  to  the 
ocean  on  the  west. 


FIG.  106.  —  PERCENTAGE  OF  TOTAL  RAINFALL  IN  THE  RAINY  SEASON.    MONTHS  OF  RAINY 
SEASON.     (U.S.  WEATHER  BUREAU.) 

Periods  of  Rainfall. — The  accompanying  chart  (Fig.  106) 
shows  the  months  of  greatest  rainfall,  and  the  percentage 
of  the  total  annual  rainfall  which  falls  during  those  months. 

The  dotted  lines  are  drawn  merely  to  roughly  separate  the  various 
regions,  and  to  show  that  the  months  and  lines  of  percentages  inclosed 
by  each  belong  together.  The  regions  between  these  must  form  transi- 
tion zones  from  one  system  to  another. 

In  most  of  the  United  States  the  period  of  greatest  rain 
is  that  usual  for  continental  localities,  except  on  the  Pacific 
coast,  where  there  is  a  winter  rainy  season. 


340  ELEMENTARY   METEOROLOGY. 

In  the  winter  time,  the  warm,  moisture-laden  winds  of 
the  Pacific  are  blown  on  to  the  colder  west  coast  of  the 
United  States,  and  precipitation  takes  place.  In  the  sum- 
mer time,  however,  this  moisture  is  rendered  still  warmer, 
and  thus  farther  removed  from  the  temperature  of  con- 
densation. 

Much  of  the  summer  rainfall  of  the  eastern  United 
States  is  of  local  character,  and  accompanies  thunder- 
storms. 

Rainfall  Types.  —  The  classification  of  the  rainfall  for  the  United 
States  by  regions  is  about  as  follows,  when  the  two  types  of  a  single 
and  a  double  period  for  the  annual  amount  of  rainfall  are  made  the 
basis  for  classification  :  — 

1.  The  Pacific  type  of  rainfall  is  characterized  by  heavy  precipita- 
tion during  midwinter,  when  the  west  and  southwest  winds  blow  from 
the  ocean  ;  and  an  almost  total  absence  of  rain  in  the  late  summer,  when 
the  winds  blow  from  the  ocean  to  the  warmer  land,  or  when  the  north 
winds  blow  from  the  dry  continent.    This  type  is  found  in  British  Colum- 
bia, Washington,  Idaho,  Oregon,  California,  Nevada,  and  western  Utah. 

2.  The  Mexican  type  of  rainfall  is  characterized  by  heavy  precipi- 
tation after  the  summer  solstice,  when  the  southeast  winds   blow  in 
the  moist  Gulf  air ;  and  a  very  dry  period  after  the  vernal  equinox. 
This  type  is  found  in  Texas,  New  Mexico,  and  most  of  Mexico.     In 
Arizona  the  type  is  complex,  resulting  from  the  meeting  of  the  Pacific 
and  Mexican  types. 

3.  The  Missouri  type  of  rainfall  is  characterized  by  light  winter 
rains,  because  the  prevailing  north  and  northwest  winds  bring  little 
moisture ;  and  heavy  rains  due  to  the  south  and  southeast  winds  blow- 
ing the  moisture  from  the  Gulf  in  the  late  spring  and  early  summer, 
when  most  of  the  annual  rainfall  occurs.     This  type  occurs  over  the 
slopes  of  the  Arkansas,  Missouri,  and  Upper  Mississippi  rivers,  and 
Lakes  Superior  and  Michigan,  and  includes  the  States  of  Montana, 
the  Dakotas,  Minnesota,  Nebraska,  Kansas,  Iowa,  Missouri,  Wisconsin, 
Illinois,  the  Territory  of  Oklahoma,  with  parts  of  Arkansas,  Texas, 
Michigan,  Indiana,  and  Indian  Territory.     This  covers  the  greatest  area 
of  any ;  and  the  time  of  occurrence  of  the  rainfall  is  most  favorable  to 
agriculture,  considering  the  total  amount  of  annual  rainfall. 


CLIMATE  OF  THE  UNITED   STATES.  341 

The  eastern  part  of  Texas  has  a  complex  rainfall,  resulting  from  the 
meeting  of  the  Mexican  and  Missouri  types. 

4.  The  Tennessee  type  of  rainfall  has  the  heaviest  rains  in  the  latter 
part  of  the  winter  or  the  early  spring,  when  moist  south  winds  from 
the  Gulf  meet  the  cold  north  winds  from  the  interior ;  and  the  least 
rain  in  mid-autumn,  when  the  cool,  dry  wind  blows  from  the  north  and 
northeast.     This  occurs  in  Tennessee,  Arkansas,  Mississippi,  eastern 
Kentucky,  western  Georgia,  Alabama,  and  Louisiana. 

5.  The  Atlantic  type  of  rainfall  has  a  somewhat  uniform  distribu- 
tion of  rain  throughout  the  year,  and  it  covers  the  Atlantic  coast  States 
except  New  England. 

6.  The  St.  Lawrence  type  of  rainfall  has  heavy  rains  in  the  late 
summer  and   autumn   months,  due  to  the   meeting   of  warm,   moist 
south,  and  cool  north  winds ;    and  a  scarcity  of  rain  in  the  spring, 
due  to  the  dry  winds  from  the  west  and  north.     It  covers  the  St. 
Lawrence  valley. 

In  N.ew  England  the  Atlantic  and  St.  Lawrence  types  meet,  and 
cause  late  summer  and  late  fall  maxima,  and  irregular  minima  in  early 
summer  and  spring  or  early  fall. 

Agriculture  Rains.  —  The  rainfall  during  the  late  spring 
and  early  summer  months  in  the  United  States  is  very 
favorable  for  agriculture  east  of  the  Rocky  Mountains,  and 
for  very  limited  regions  to  the  west.  A  well-distributed 
rainfall  of  over  two  inches  per  month  is  found  east  of 
about  the  looth  meridian,  in  April;  east  of  the  Ii5th 
meridian  (central  Idaho)  in  the  northern  United  States, 
and  east  of  the  iO2d  meridian  (western  Texas)  in  the 
southern  United  States,  in  May;  and  east  of  the  H5th 
meridian  at  the  north  (central  Montana),  the  iO2d  merid- 
ian at  the  center  (eastern  Colorado),  and  the  looth 
meridian  at  the  south  (central  Texas),  in  June.  It  is  to 
these  widely  distributed  sufficient  rainfalls  that  the  agri- 
cultural prosperity  of  the  United  States  is  largely  due. 
In  most  of  the  centra!  and  eastern  United  States  the 
rainfall  is  between  two  and  four  inches  per  month.  Where 


342  ELEMENTARY   METEOROLOGY. 

it  is  over  six  inches  per  month  in  limited  regions,  mostly 
near  the  Gulf  coast,  the  excess  is  due  to  a  few  heavy 
rains. 

Excessive  Rains  are  those  which  are  injurious  to  plant 
growth,  and  which  cause  damaging  floods.  When  the 
rainfall  exceeds  two  inches  in  a  day,  or  ten  inches  in  a 
month,  it  becomes  excessive.  Such  rains  are  most  fre- 
quent on  the  North  Pacific  coast  from  January  to  March, 
on  the  South  Atlantic  coast  in  summer,  on  the  Gulf  of 
Mexico  in  the  spring,  and  in  the  Tennessee  and  Kentucky 
region  from  the  late  winter  to  early  summer.  Occasionally 
these  excessive  rainfalls  occur  north  of  the  Potomac  River, 
chiefly  in  the  late  summer  or  early  fall. 

An  excessive  monthly  rainfall  of  nea-rly  42  inches  has 
occurred  in  northern  California,  one  of  37  inches  in  Lou- 
isiana, one  of  over  28  inches  in  North  Carolina,  and  one  of 
over  22  inches  in  New  Jersey. 

The  greatest  daily  rainfalls  (in  24  hours)  do  not  usually 
exceed  10  inches,  with  occasionally  double  that  amount 
during  two  successive  days.  The  chart,  Fig.  107,  shows 
the  greatest  daily  rainfall  in  various  regions  of  the  United 
States. 

On  June  15  and  16,  1886,  and  within  a  period  of  24  hours,  nearly 
21.5  inches  of  ram  fell  at  Alexandria,  La.  On  Feb.  11-13,  1886, 
there  occurred  a  remarkable  storm  in  New  England,  in  which  precipi- 
tation was  excessive  over  a  large  extent  of  territory.  The  amount 
was  12.4  inches  at  Canton,  Conn. ;  and  it  averaged  more  than  7  inches 
over  1,500  square  miles,  and  more  than  5  inches  over  5,000  square 
miles. 

Rain  frequently  falls  at  the  much  more  rapid  rate  of  from  5  to  18 
inches  per  hour  during  short  intervals  of  time.  Such  rainfalls  usually 
last  from  2  or  3  to  15  minutes,  and  rarely  for  so  long  as  half  an 
hour.  In  Galveston,  Tex.,  in  June,  1871,  there  was  a  rainfall  of  nearly 
4  inches  in  14  minutes ;  but  usually  the  amount  during  these  intense 


CLIMATE   OF  THE   UNITED    STATES.  343 

rainfalls  is  about  2  inches.  We  do  not  know  much  about  the  amount 
of  rainfall  in  the  cloud-bursts,  except  from  the  overflowing  of  creeks 
and  the  damage  which  is  done. 


FIG.  107.  —  HEAVIEST  DAILY  RAINFALLS  (U.S.  WEATHER  BUREAU). 

Snowfall  in  the  United  States.  —  Snow  falls  with  greater 
or  less  frequency  in  almost  all  parts  of  the  United  States. 
Since  snowfall  depends  on  the  temperature,  snow  occurs 
in  very  low  latitudes  if  the  altitude  is  sufficiently  great ; 
but  it  is  almost  unknown  on  the  Florida  peninsula  and 
on  the  California  coast  below  the  35th  parallel  Taking 
the  country  as  a  whole,  snow  seldom  remains  unmelted  on 
the  ground  south  of  about  latitude  33°  at  low  altitudes  ; 
and  on  the  eastern  coast  this  limiting  latitude  is  about 
31°,  while  on  the  western  coast  it  is  about  37°. 

The  region  of  greatest  snowfall  would  naturally  be  that 
in  which  the  greatest  amount  of  precipitation  occurs  in 
winter,  but  it  is  also  necessary  that  the  temperature  be 
low  enough  to  convert  the  moisture  into  snow.  The  total 
average  annual  amount  of  snowfall  in  inches,  for  the 


(344) 


CLIMATE  OF  THE  UNITED   STATES,  345 

7-year   period    1884-91,   is    shown  on  the    accompanying 
chart  (Fig.  108). 

The  lines  connect  the  regions  of  equal  snowfall,  and  the  attached 
figures  indicate  the  amounts  of  snowfall  in  inches.  The  snowfalls  rep- 
resented on  this  chart  refer  to  the  low  lands  and  plains  in  which  the 
Larger  towns  are  situated.  Where  there  are  mountains,  the  present 
figures  might  have  to  be  increased  even  several  fold  in  order  to  repre- 
sent the  snowfall  at  higher  altitudes  than  their  bases. 

In  general,  for  the  low  lands,  the  region  of  greatest 
snowfall  is  from  northern  Michigan  (where  it  is  130  inches) 
eastward  along  the  Canadian  frontier.  To  the  southward 
'of  this  region  there  is  a  rapid  decrease  to  20  inches  at 
about  the  latitude  of  St.  Louis,  and  then  the  decrease  is 
very  slow  until  the  practical  disappearance  of  snow  near 
the  Gulf  coast.  On  the  Atlantic  coast  the  snowfall  is  60 
inches  at  the  north,  and  diminishes,  at  first  rapidly  and 
then  more  gradually,  to  the  limit  of  practical  disappearance 
on  the  Carolina  coast.  To  the  west  of  the  region  of 
maximum  snowfall  in  northern  Michigan,  there  is  a  dimi- 
nution to  nearly  30  inches  in  the  Missouri  valley,  and  then 
an  increase  to  50  inches  on  the  high  plateaus  from  Utah 
to  Montana,  with  a  decrease  farther  west  towards  the 
Pacific  Ocean.  In  the  whole  southwest  United  States 
the  snowfall  is  slight,  except  on  the  mountains.  On  the 
Pacific  coast  the  snowfall  is  less  than  10  inches  at  the 
north,  and  decreases  to  less  than  i  inch  at  San  Fran- 
cisco ;  while  south  of  San  Francisco  snow  seldom  falls. 
Along  the  Mississippi  River  there  is  a  decrease  from  40 
inches  at  the  extreme  north,  to  20  inches  at  St.  Louis, 
and  2  inches  at  Vicksburg.  The  greatest  recorded  annual 
amounts  of  snowfall  are  in  eastern  California  and  western 
Nevada,  where  they  reach  several  hundred  inches.  At 


346 


ELEMENTARY   METEOROLOGY. 


Summit,  Cal.,  on  the  Central  Pacific  Railroad,  the  snowfall 
averaged  378  inches  per  year  during  a  period  of  ten  years. 

For  the  greater  portion  of  the  United  States  it  may  be 
said  that  in  the  northern  half  the  snowfall  exceeds  20 
inches,  and  in  the  southern  half  is  less  than  20  inches. 

Frequency  of  Precipitation.  —  The  distribution  of  the 
number  of  days  with  precipitation  (rainy  and  snowy  days) 
follows  in  part  the  amounts  of  precipitation,  and  in  part 


FIG.  109.  —  AVERAGE  NUMBER  OF  DAYS  DURING  THE  YEAR  WITH  PRECIPITATION 
(U.S.  WEATHER  BUREAU). 

the  distribution  through  the  year.  For  the  whole  year, 
along  about  the  central  meridian  of  the  United  States, 
there  is  precipitation  on  about  100  days  (Fig.  109).  To  the 
eastward  there  is  an  increase  towards  the  Atlantic  Ocean, 
with  a  local  maximum  over  the  lower  Great  Lakes.  To  the 
westward  there  is  an  increase  towards  the  Pacific  Ocean  in 
the  northern  part,  and  a  decrease  in  the  southern  part. 

In   the  western    United    States,   where  the  seasons  of 
rainfall  are  strongly  marked,  the  number  of  rainy  days  is 


CLIMATE   OF  THE   UNITED    STATES*  347 

greatest  in  the  region  of  greatest  rainfall,  and  least  in  the 
region  of  least  rainfall.  In  the  eastern  United  States, 
however,  the  region  of  greatest  number  of  days  is  at  the 
north,  where  the  rainfall  is  more  equably  distributed  over 
the  whole  year,  and  where  the  passage  of  barometric 
minima  is  most  frequent. 

The   Greatest  Number   of  Consecutive   Days  with  Pre- 
cipitation is  shown  on  the  chart,  Fig.   no.     In  general, 


FIG.  no. —  GREATEST  NUMBER  OF  CONSECUTIVE  DAYS  WITH  PRECIPITATION 
(U.S.  WEATHER  BUREAU). 

the  regions  of  greatest  rainfall  have  the  greatest  number 
of  consecutive  days  with  precipitation.  For  the  main  part 
of  the  United  States,  the  number  of  days  is  from  10  to 
20 ;  but  on  the  northwestern  coast,  where  there  is  such 
a  pronounced  rainy  season,  with  steady  winds  from  the 
ocean,  there  is  an  increase  to  nearly  40  days  ;  while  in  the 
northeastern  United  States  in  the  St.  Lawrence  valley, 
where  the  cyclonic  areas  are  most  frequent,  the  number 
of  days  increases  to  over  30. 


348 


ELEMENTARY    METEOROLOGY. 


The  Greatest  Number  of  Consecutive  Days  without  Pre- 
cipitation is  of  importance  as  signifying  the  duration  of 
droughts,  and  is  shown  on  the  chart,  Fig.  in.  The  more 
equable  the  distribution  of  precipitation  through  the  year, 
the  less  the  number  of  consecutive  days  without  it.  In 
nearly  the  whole  of  the  eastern  half  of  the  United  States, 
the  greatest  period  of  uninterrupted  drought  is  from  15  to 
30  days,  except  in  the  extreme  southeastern  part,  where  it 
varies  from  30  to  about  50  days.  There  is  an  increase  in 


FIG.  in.  —  GREATEST  NUMBER  OF  DAYS  WITHOUT  PRECIPITATION  (DROUGHT) 
(U.S.  WEATHER  BUREAU). 

the  number  of  days  from  the  center  of  the  United  States 
towards  the  northwest,  to  from  30  to  60  days,  and  towards 
the  west  and  southwest  to  over  150  days. 

Absolute  Humidity.  —  The  absolute  amount  of  water 
contained  in  the  air  is  shown  on  the  charts,  Figs.  112, 
113,  which  give  the  number  of  grains  contained  in  each 
cubic  foot  of  surface  air  for  the  various  regions  of  the 
United  States.  Since  the  possible  amount  of  water  in- 


CLIMATE  OF  THE   UNITED   STATES. 


349 


creases  with  the  increase  of  the  temperature,  the  distribu- 
tion follows  somewhat  after  that  of  the  temperature,  and 
is  least  in  winter  and  greatest  in  summer ;  but  the  dis- 
tance and  accessibility  of  the  supply  of  moisture,  and  the 
direction  of  the  wind,  also  enter  into  this  distribution. 

In  the  Winter  (Fig.  112)  there  is  a  region  of  least  mois- 
ture in  the  region  of  lowest  temperature  in  the  north 
central  United  States  ;  and  there  is  an  increase  towards 
the  east,  south,  and  west,  in  the  direction  of  increase  of 


FIG.  112. — AVERAGE  ABSOLUTE  HUMIDITY  IN  MIDWINTER,  JANUARY  (AFTER  GRKELY). 

temperature,  and  roughly  in  proportion  to  this  increase  of 
temperature. 

In  the  Summer  (Fig.  113)  the  humidities  are  much  in- 
creased over  those  of  winter,  and  the  increase  is  propor- 
tionally the  greater  where  there  is  the  greater  annual 
range  of  temperature,  although  the  increase  in  grains  is 
nearly  the  same  in  all  parts.  The  region  of  least  moisture 
is  now  shifted  to  the  high  plateau  west  of  the  Rocky 
Mountains,  where,  with  the  region  to  the  west  of  it,  the 


350 


ELEMENTARY  METEOROLOGY. 


average  temperatures  are  lowest.  From  thence  there  is 
an  increase  towards  the  higher  temperatures  to  the  east, 
southeast,  and  south.  In  the  southwestern  part  of  the 
United  States  the  moisture  would  undoubtedly  become 


FIG.  113.  —  AVERAGE  ABSOLUTE  HUMIDITY  IN  MIDSUMMER,  JULY  (AFTER  GREELY). 

greater  if  it  were  not  for  the  mountain  ranges  cutting  off 
the  winds,  and  the  unfavorable  wind  direction  with  regard 
to  the  supply  of  moisture. 

The  Relative  Humidity  (Fig.  114),  depending  as  it  does 
on  the  absolute  moisture  and  the  height  of  the  tempera- 
ture above  that  necessary  for  condensation,  is  least  in  the 
southwestern  United  States,  where  the  amount  of  moisture 
is  low  and  the  temperature  is  high.  There  is  an  increase 
in  all  directions  from  this  region  to  the  limits  of  the  United 
States.  This  increase  is  fairly  symmetrical,  although  not 
with  the  same  rapidity  on  all  sides.  The  relative  humid- 
ity, while  lowest  in  summer  and  highest  in  winter,  is  so 
irregularly  distributed  throughout  the  year,  that  midwinter 
and  midsummer  charts  are  not  given. 


352  ELEMENTARY   METEOROLOGY. 

The  average  relative  humidity  for  the  year  (Fig.  1 14)  is 
distributed  over  the  United  States  about  as  follows  :  On 
the  Atlantic  and  Pacific  coasts  there  is  a  relative  humidity 
of  abo.ut  So%  on  the  northern  coast,  which  decreases  to 
a  little  below  75  %  on  the  southern  coast ;  on  the  coasts 
of  the  Great  Lakes  and  the  Gulf  of  Mexico  it  is  about 
75  %  ;  in  the  low  eastern  half  of  the  inland  United  States 
it  is  about  70%,  except  in  the  upper  Ohio  River  valley 
and  western  Virginia,  where  it  decreases  to  65  %  ;  in  the 
elevated  western  half  of  the  inland  United  States  it  is 
below  60%,  and  decreases  gradually  to  40%  in  southwest 
Arizona. 

Degree  of  Cloudiness.  —  The  observations  of  the  degree 
of  cloudiness  are  made  on  a  scale  of  from  o  for  clear 
weather  to  10  for  an  entirely  overcast  sky,  but  the  aver- 
age results  are  as  usual  expressed  in  percentage  of  com- 
plete cloudiness.  Thus  50  %  of  cloud  signifies  that  the 
sky  is  half  covered  with  cloud. 

The  degree  of  cloudiness,  which  depends  on  the  relative 
humidity,  varies  with  the  season  of  the  year  somewhat 
irregularly,  but  is  in  general  greatest  in  winter  and  least 
in  summer  ;  but  the  maximum  occurs  in  the  spring  months 
in  the  Rocky  Mountain  region,  and  over  the  Great  Plains 
at  the  center  of  the  continent. 

Cloudiness  in  January  (Fig.  115).  —  In  the  extreme  north- 
west of  the  United  States  there  is  a  cloudiness  of  over  70  %, 
which  decreases  towards  the  south  to  40%  on  the  south 
Pacific  coast,  towards  the  southeast  to  between  30  %  and 
40  %  for  nearly  the  whole  of  the  southwestern  part  of  the 
United  States,  and  towards  the  east  nearly  to  40  %  on  the 
northern  Great  Plains.  At  nearly  the  longitudinal  center 
of  the  continent  there  is  a  cloudiness  of  about  50%,  and 
to  the  eastward  of  this  there  is  a  quite  rapid  increase  in  the 


CLIMATE  OF  THE   UNITED    STATES. 


353 


northern  part  to  over  70%  on  the  Lower  Lake  region, 
with  a  decrease  to  less  than  60  Sfo  on  the  North  Atlantic 
coast ;  while  in  the  southeastern  part  of  the  United  States 
there  is  at  first  an  increase  to  about  60  %,  and  then  a  slight 
decrease  again  as  the  coast  is  approached.  In  Florida  the 
percentage  falls  below  50%,  except  on  the  eastern  coast. 

The  central  line  of  the  region  of  least  cloudiness  passes  from  south- 
ern Arizona  in  a  northeasterly  direction  near  the  northwest  corner  of 
New  Mexico,  the  western  part  of  Colorado,  the  southwest  corner  of 


FIG.  115. — AVERAGE  CLOUDINESS  IN  MIDWINTER,  JANUARY  (AFTER  GKEELY). 
(Scale:  o  =  clear;  100  =  completely  cloudy.) 

South  Dakota,  and  the  northeast  corner  of  North  Dakota.  To  the  east 
and  west  of  this  line  there  is  a  very  rapid  increase  in  the  cloudiness 
in  the  northern  part,  and  very  much  smaller  increase  in  the  southern 
part,  of  the  country. 

Cloudiness  in  August  (Fig.  116). --The  cloudiness  in 
August  is  least  in  the  Sierra  Nevada  region,  where  it  is 
less  than  10%,  and  is  greatest  on  the  southern  Pacific 
coast,  the  Atlantic  coast,  and  in  a  small  region  in  north- 

WALDO    METEOR. —  21 


354 


ELEMENTARY   METEOROLOGY. 


western  New  Mexico,  where  it  varies  from  45  %  to  50  %  of 
total  cloudiness. 

In  the  northern  half  of  the  United  States  there  is  an 
uninterrupted  increase  in  the  cloudiness  from  20%  in  the 
western  part  (Idaho)  to  about  45  %  in  the  region  of  the 
Great  Lakes,  whence  it  varies  little  to  the  Atlantic  coast. 
The  increase  is  slow  west  of  the  Rocky  Mountains,  but 
east  of  them  is  much  more  rapid.  In  the  southern  half 
of  the  United  States  there  is  a  cloudiness  of  45  %  on  the 


FIG.  116. — AVERAGE  CLOUDINESS  IN  MIDSUMMER,  AUGUST  (AFTER  GREELY). 
(Scale:  o  =  clear;  100  =  completely  cloudy.) 

Pacific  coast,  and  a  rapid  decrease  to  10  %  in  the  near 
inland  (central  California),  a  local  increase  to  45  %  or  50% 
in  New  Mexico  and  Colorado,  then  a  decrease  to  35% 
at  the  looth  meridian,  and  thence  eastward  a  steady  in- 
crease to  50  %  on  the  eastern  coast. 

At  the  meridional  center  of  the  continent,  the  cloudiness 
is  from  35  %  to  40  %,  and  in  general  decreases  towards  the 
west,  and  increases  towards  the  east,  by  about  1 5  %. 


CLIMATE  OF  THE  UNITED   STATED. 


355 


Winds. 

Average  Wind  Velocity  for  the  Year,  in  Miles  per  Hour 
(Fig.  117). — -If  the  average  amount  of  wind  which  blows 
during  the  entire  year  were  evenly  distributed  through  all 
the  hours  of  the  year,  it  would  make  a  uniform  velocity  of 
about  13  miles  per  hour  on  the  North  Atlantic  coast,  14  on 
the  central  coast,  and  probably  decrease  to  11  or  12  on  the 
southern  coast.  On  the  shores  of  the  Great  Lakes,  it 


FIG.  117. — AVERAGE  WIND  VELOCITIES  FOR  THE  VEAR  (IN  MILES  PER  HOUR). 

would  be  about  1 1  miles  per  hour ;  and  on  the  Gulf  coast, 
in  the  western  part  about  12,  but  in  the  eastern  part  about 
10,  miles  per  hour.  On  the  Pacific  coast,  a  velocity  of 
ii  miles  per  hour  at  the  north  decreases  to  probably  8 
miles  or  less  per  hour  at  the  south.  On  the  Great  Plains, 
extending  from  northern  Dakota  to  Texas,  the  velocities 
would  be  about  10  miles  per  hour,  with  a  maximum  of 
ii  in  central  Kansas.  To  the  west  of  about  the  looth 


356  ELEMENTARY   METEOROLOGY. 

meridian  there  is  a  decrease  to  6  miles  per  hour  on 
the  Pacific  slope  along  the  region  of  central  California, 
Oregon,  and  Washington.  Towards  the  east  from  the 
looth  meridian  there  is  a  decrease  to  8  miles  per  hour 
in  the  northeastern  part  of  the  United  States  (except 
directly  along  the  Lakes),  and  to  less  than  7  miles  per 
hour  for  most  of  the  region  south  of  the  Ohio  and  east 
of  the  Mississippi  River.  The  velocities  are  thus  the 
greatest  on  the  coasts  and  treeless  prairies,  and  least  in- 
land and  in  the  mountainous  and  wooded  regions. 

Month  of  Maximum  and  of  Minimum  Wind.  — The  month 
of  maximum  wind  varies  somewhat  in  the  different  regions 
of  the  United  States,  but  usually  occurs  in  March  in  the 
eastern  half,  and  in  April  or  a  little  later  in  the  western 
half,  of  the  country.  The  time  of  minimum  is  not  quite 
so  irregular,  and  occurs  usually  in  August. 

At  low  altitudes  in  the  eastern  United  States,  away 
from  the  coast,  and  as  far  west  as  eastern  Kansas,  the 
maximum  wind  occurs  in  March  ;  but  where  the  stations 
have  a  greater  altitude,  or  are  directly  on  the  coast,  it 
occurs  in  an  earlier  month,  perhaps  as  early  as  Decem- 
ber, or  even  November.  In  the  region  to  the  west  of  the 
looth  meridian  (central  Kansas),  and  extending  to  the 
Pacific  slope,  the  month  of  maximum  is  irregular,  and 
at  moderate  altitudes  occurs  in  April  or  sometimes  in 
May  or  June  ;  but  at  very  high  altitudes  there  is  a  return 
to  the  beginning  of  the  year,  which  was  noticed  in  the 
east.  On  the  Pacific  slope  the  months  are  not  the  same 
in  different  localities.  On  the  southern  coast  the  maxi- 
mum occurs  in  April,  and  the  minimum  in  November  ; 
on  the  central  coast  the  maximum  is  in  July,  and  the 
minimum  in  November.;  on  the  northern  coast  the  maxi- 
mum is  in  January. 


CLIMATE  OF  THE  UNITED   STATES.  357 

Amount  of  Maximum  and  Minimum  Monthly  Winds.  — 

The  average  amount  of  wind  for  the  months  of  greatest 
and  least  wind  is  about  17  miles  per  hour  for  the  greatest, 
and  9  for  the  least,  on  the  North  Atlantic  coast ;  probably 
14  or  15  for  the  greatest,  and  9  to  12  for  the  least,  on  the 
South  Atlantic  coast ;  about  11  or  12  for  the  greatest,  and 
6  for  the  least,  on  the  South  Pacific  coast ;  and  16  for  the 
greatest,  and  7  for  the  least,  on  the  North  Pacific  coast. 
On  the  Great  Lakes  the  average  velocity  is  about  14  miles 
per  hour  for  the  greatest,  and  9  for  the  month  of  least  wind  ; 
while  on  the  Gulf  coast  it  is  15  for  the  greatest  and  10  for 
the  least  at  the  western  part  (Texas),  and  only  1 1  for  the 
greatest  and  8  for  the  least  at  the  eastern  part  (Florida). 
These  numbers  apply  to  the  immediate  shore ;  and  even 
a  single  mile  inland  the  velocities  are  much  reduced,  prob- 
ably as  much  as  33  %  near  the  ground  even  if  the  inter- 
vening land  is  level. 

Away  from  the  immediate  coast  the  average  monthly 
wind  velocities  reach  about  9  miles  per  hour  for  the  great- 
est and  6  for  the  least  in  the  southeastern  part  of  the 
United  States ;  1 1  for  the  greatest  and  6  for  the  least  in 
the  northeastern  part ;  but  increase  with  westward  progress 
to  13  miles  per  hour  for  the  greatest  and  9  for  the  least 
on  the  Great  Plains  from  North  Dakota  to  southern  Texas, 
and  decrease  still  farther  to  the  west  to  8  for  the  greatest 
and  6  for  the  least  in  the  southwest  United  States,  and 
1 1  miles  per  hour  for  the  greatest  and  6  for  the  least  in 
the  northwest  United  States. 

The  difference  between  the  average  amount  of  wind  in  the  windiest 
and  calmest  months  of  the  year,  expressed  in  percentage  of  the  average 
wind,  is  about  as  follows :  On  the  North  Atlantic  coast  region,  from 
50  %  to  60  % ;  on  the  central  coast,  40  % ;  and  on  the  southern  coast, 
30%;  on  the  Upper  Lakes,  30%,  and  on  the  Lower  Lakes,  40%;  on 


358  ELEMENTARY   METEOROLOGY. 

the  Gulf  of  Mexico  coast,  40  % ;  on  the  Pacific  coast,  70  %  at  the  north, 
60%  at  the  center,  and  probably  not  over  40  %  at  the  south.  Inland, 
east  of  the  Rocky  Mountains,  there  is  an  increase  from  30  %  to  50  % 
from  the  northern  boundary  of  the  United  States  towards  the  south,  and 
towards  the  southeast  as  far  as  the  western  boundary  of  North  Carolina, 
but  still  farther  to  the  southeast  there  is  a  decrease  to  30  %.  To  the 
west  of  the  Great  Plains  there  is  an  increase  to  about  60  %  along  the 
Rocky  Mountain  region,  but  it  decreases  again  to  only  20  %  in  central 
Nevada,  whence  it  gradually  increases  again  towards  the  coast. 

A  marked  peculiarity  of  this  feature  is  that  the  same  percentages 
found  on  the  coast  sometimes  extend  far  inland ;  and  consequently 
these  ratios  expressed  in  percentages  hold  good  for  the  region  in  which 
they  occur,  irrespective  of  a  land  or  water  surface. 

Maximum  Wind  Velocities.  —  The  maximum  velocities 
attained  in  individual  wind  storms  are  greatest  in  regions 
of  tornadoes  and  severe  thunder  squalls,  but  these  winds 
are  of  short  duration.  Strong  winds  which  blow  for  a 
considerable  length  of  time,  say  for  over  five  minutes, 
have  velocities  of  from  50  to  70  miles  per  hour  inland,  and 
from  70  to  90  miles  per  hour  on  the  coasts  of  the  United 
States.  Inland  the  velocities  are  less  at  the  south  than 
farther  north ;  but  on  the  southern  Atlantic  and  Gulf 
coasts  the  hurricane  winds  are  as  strong  as  those  ex- 
perienced on  the  northern  coast.  On  the  Pacific  coast 
the  velocities  increase  from  less  than  40  miles  per  hour 
at  the  south  to  over  90  at  the  north.  The  southwest 
United  States  is  the  region  of  least  maximum  winds. 

Wind  Direction. — The  direction  of  the  wind  is  in  gen- 
eral towards  the  east  with  the  -primary  air  currents,  but 
it  suffers  interruption  from  monsoon  effects  and  the  pas- 
sage across  the  country  of  areas  of  high  and  low  air  pres- 
sure. The  average  wind  directions  for  any  length  of  time 
should  conform  to  the  distribution  of  the  air  pressure  dur- 
ing that  time  ;  and  this  distribution,  in  extreme  seasons, 


CLIMATE  OF  THE  UNITED   STATES, 


359 


over  the  United  States  is  probably  about  as  given  in 
Figs.  1 1 8,  119;  but  there  is  considerable  uncertainty 
about  the  pressures  in  the  elevated  western  region. 

There  is  a  strong  tendency  for  winds  of  a  monsoon  char- 
acter to  arise,  especially  in  the  great  central  region  extend- 
ing from  the  Gulf  of  Mexico  to  Canada ;  but  the  frequency 
and  magnitude  of  the  cyclonic  areas  prevent  these  from 
becoming  so  well  marked  as  they  would  be  in  lower  lati- 


FIG.  118.  —  NORMAL  AIR  PRESSURE  AT  SEA  LEVEL,  AND  NORMAL  WIND  DIRECTION,  FOR 
JANUARY  (U.S.  WEATHER  BUREAU  AND  HAZEN). 

tudes.     We   can   consider   in    detail   only  the  midwinter 
month  of  January  and  the  midsummer  month  of  July. 

The  Average  Wind  Direction  for  January  is  shown  on 
Fig.  1 1 8.  The  surface  wind  directions,  in  the  greater 
part  of  the  United  States,  are  mainly  the  result  of  the 
anticyclonal  motion  around  the  region  of  high  air  pressure 
over  the  interior  of  the  continent,  acting  on  the  general  cur- 
rent towards  the  east.  The  low  pressure  at  the  northeast 
strongly  affects  the  New  England  winds.  Along  the  4Oth 


360  ELEMENTARY   METEOROLOGY. 

parallel  the  wind  direction  is  towards  the  east  all  the  way 
across  the  continent,  except  on  the  high  plateau  of  Utah, 
where  it  is  northerly.  To  the  north  of  this  parallel  we 
find  in  the  extreme  northwest  in  Washington  a  wind 
towards  the  west ;  in  Idaho  it  swerves  around  towards  the 
north,  in  Montana  it  has  turned  towards  the  east,  and  in 
the  Dakotas  towards  the  southeast ;  and  more  to  the  east- 
ward, in  the  Great  Lake  Region,  there  is  a  tendency  for 
the  winds  to  turn  in  a  direction  to  the  north  of  east,  but 
in  New  England  there  is  a  bend  again  towards  the  south- 
east, owing  to  the  area  of  low  pressure  at  the  northeast. 
Between  the  4Oth  and  3Oth  parallels  there  is  a  wind  towards 
the  south  in  the  Western  States,  towards  the  southwest  in 
the  Central  States,  and  towards  the  southeast  in  the  East- 
ern States.  South  of  the  3Oth  parallel  the  wind  is  towards 
the  west  in  the  eastern  part,  but  swerves  around  towards 
the  south  in  the  Western  States. 

The  Average  Wind  Direction  for  July  is  shown  in  Fig.  1 19. 
The  area  of  low  pressure  probably  centering  over  Arizona 
and  Utah  controls  the  direction  of  the  surface  wind  for 
the  region  west  of  the  looth  meridian,  except  at  the  ex- 
treme north,  where  the  wind  direction  is  towards  the  east ; 
so  that  in  the  western  United  States  there  is  a  wind 
towards  the  south  at  the  north-central  part,  towards  the 
west  at  the  eastern  part,  towards  the  east  at  the  western 
part,  while  at  the  southern  part  the  wind  is  towards  north 
and  east.  The  area  of  low  pressure  in  the  northeastern 
United  States  (and  Lower  Canada)  gives  the  easterly  direc- 
tion to  the  wind  in  that  region.  The  area  of  moderately 
high  pressure  over  the  southeastern  United  States  fur- 
nishes winds  blowing  outwards  towards  the  areas  of  low 
pressures  to  the  west  and  to  the  north.  In  the  south- 
eastern part  of  the  United  States  the  winds  are  towards 


CLIMATE  OF  THE  UNITED   STATES.  361 

the  north ;  in  the  eastern  section  of  this  region  they  in- 
cline towards  the  east  of  north  (towards  the  area  of  low 
pressure  in  the  northeastern  United  States),  and  in  the 
western  part  towards  the  west  of  north  (towards  the  area 
of  low  pressure  in  the  western  United  States). 


FIG.  119. —  NORMAL  AIR  PRESSURE  AT  SEA  LEVEL,  AND  NORMAL  WIND  DIRECTION,  FOR 
JULY  (U.S.  WEATHER  BUREAU  AND  HAZEN). 

Rain-bearing  Winds.  —  The  direction  of  winds  most 
likely  to  be  followed  by  precipitation,  for  the  whole  year, 
is  given  on  the  chart,  Fig.  120,  the  limits  being  marked 
by  the  arrows.  These  wind  directions  depend  a  good  deal 
on  the  direction  of  the  water  supply,  and  the  nature  of  the 
intervening  country ;  and  they  vary  somewhat  from  month 
to  month.  Such  charts  are  very  useful  in  making  predic- 
tions of  rain  in  weather  forecasting. 

Destructive  Storms  in  the  United  States. —  The  greatest 
number  of  destructive  storms,  which  are  usually  of  a  thun- 
der-squall or  tornadic  character,  occurs  in  the  months  of 
June  and  July.  They  occur  more  frequently  in  the  spring 


362 


ELEMENTARY   METEOROLOGY. 


than  in  the  fall ;  and  there  is  a  retardation  with  the  advance 
of  the  season  and  with  increase  of  latitude.  Thus,  in  the 
southern  part  of  the  United  States,  tornadoes  are  frequent 
in  the  spring,  but  in  the  northern  part  they  are  most  fre- 
quent when  summer  is  well  advanced.  They  seldom  occur 
west  of  the  looth  meridian,  since  the  air  farther  west  is 


FIG.  120. —  WIND  DIRECTION  MOST  LIKELY  TO  BE  FOLLOWED  BY  RAIN. 

too  dry  for  their  formation.  The  greatest  number  of 
destructive  storms  occur  in  Illinois,  Iowa,  and  Pennsyl- 
vania, and  the  least  number  in  California. 

The  chart,  Fig.  121,  shows  the  average  annual  number 
of  thunderstorms  in  the  United  States,  and  the  months 
of  their  greatest  frequency.  It  is  seen  that  for  the  great 
central  region,  including  the  Mississippi  River  and  Ohio 
River  valleys,  the  month  of  greatest  frequency  is  June ; 
but  there  is  a  retardation  of  about  two  months  with 
approach  to  the  eastern  and  southern  coasts,  and  also  with 
the  progress  into  the  region  of  deficient  moisture  in  the 
western  part  of  the  United  States. 


CLIMATE  OF  THE  UNITED   STATES. 


363 


The  region  of  greatest  frequency  of  thunderstorms  is  in 
the  southeastern  part  of  the  United  States  near  the  coast, 
where,  on  the  average,  40  occur  during  the  year.  There 
is  a  decrease  in  all  directions,  and  the  number  is  reduced 
to  about  20  at  about  the  io$th  meridian  on  the  west,  at  the 
latitude  of  the  northern  boundary  of  the  United  States  on 


FIG.  121.  —  MONTHS  OF  MAXIMUM  FREQUENCY,  AND  AVERAGE  ANNUAL  NUMBER,  OF  THUN- 
DERSTORMS (U.S.  WEATHER  BUREAU). 

the  north,  and  at  the  Virginia  coast  on  the  northeast.     On 
the  New  England  coast  only  10  occur. 

We  have  now  touched  on  the  main  features  of  the  climate 
of  the  United  States.  A  study  of  charts  will  reveal  many 
details  which  it  has  not  been  possible  to  mention  specifi- 
cally, and  the  pupil  is  recommended  to  make  frequent 
study  of  these  maps,  and  thus  become  familiar  with  the 
characteristics  of  the  climate  in  the  various  sections  of  our 
countryc 


INDEX. 


Absorption,  thermal,  20. 

of  solar  rays  by  atmosphere,  27. 
Accidental  change,  17. 
Adiabatic,  49. 

cooling  of  moist  air,  143. 

expansion,      formation      of      cloud 
through,  135. 

heating  and  cooling  of  air,  49, 183-185. 
Africa,  climate  of,  307. 
Agricultural  rains  in  United  States,  341. 
Air  (see  also  Atmosphere),  7-14. 

composition  of,  7,  8,  12. 
Air  currents  (see  also  Winds),  101. 

ascending  and  descending,  184. 

breaking  of,  into  eddies,  231. 

effect  of  local  high  temperature  on, 

232. 
Air,  density  of,  75,  234. 

energy  of,  182. 

expansion  and  contraction  of,  33. 

heat  in,  20,  180. 

heat  of  earth  retained  by,  29 

microscopical  impurities  in,  9. 

moisture  of  (see  Moisture),  118-142. 
Air    motions    (see    also    Winds),    101, 
187-268. 

between  northern  and  southern  hemi- 
spheres, 205. 

easterly,  at  high  latitudes,  198,  205. 

effect  on  air  pressure,  201. 

effect  of  earth's  rotation  on,  195-199. 

equatorial,  192. 

horizontal,  101,  180. 

in  whirls,  213. 

poleward,  191. 

retardation  of,  in,  199. 

velocities  of,  205. 

vertical,  102,  180,  192,  200. 

westerly,  at  low  latitudes,  198,  205. 


Air  pressure  (see  Pressure),  73-101. 
Air,  radiation  of  heat  from,  183. 

temperature   of  (see  Temperature), 
19-70. 

transparency  of,  166. 

waves,  117. 

weight  and  pressure  of,  10,  13. 
Altitude,  14,  15. 

determination  of  differences  in,  79. 

effect  on  climate,  301. 
Alto-cumulus  clouds,  130,  131. 
Alto-stratus  clouds,  130,  131. 
Anemometer,  104. 
Anticyclones,  214-216,  234-239. 

classes  of,  234. 

dimensions  of,  236,  238. 

excessive  pressure  in,  238. 

in  United  States,  314-317. 

in  tropics,  239. 

irregular  movement  of,  282-285. 

moisture  in,  236. 

paths  of,  in  United  States,  283,  314- 
316. 

permanent,  240. 

pressure  of  air  in,  236,  238. 

relation  to  cyclones,  235,  238-240. 

ring  of,  in  low  latitudes,  235. 

rotation  of,  214-216. 

temperature  in,  237. 

velocity  of,  237. 

weather  conditions  of,  273. 

winds  in,  237. 

Aqueous  vapor  (see  Vapor). 
Argon,  7,  8. 
Asia,  climate  of,  308. 
Atlantic  Ocean,  winds  of,  113-115. 
Atmosphere  (see  also  Air),  7. 

vertical  thickness  of,  12-14. 
Atmospheric    circulation,    general,   102, 
187-212. 


365 


366 


ELEMENTARY   METEOROLOGY. 


Atmospheric  circulation,  cause  of,  189. 

diagram  of,  188. 

secondary,  213-240. 

viewed  as  a  whole,  204. 

without  rotation  of  earth,  191, 
Atmospheric      disturbances,      irregular 
local,  241. 

electricity,  175-179. 

moisture  (see  Moisture),  118-142. 

optics,  166-175. 
Aurora.  175. 

Avalanche  winds,  103,  267. 
Average  condition,  17. 
Axis,  inclination  of  earth's,  22,  23. 


B 

Bacteria,  9. 
Barometer,  75-77. 

aneroid,  77. 

height  or  reading,  76. 
Barometric  gradients,  100,  201,  202. 

maxima,  88. 

minima,  88. 

pressure   (see   under   Pressure),  73- 

101. 

Blizzards,  264. 
Bora,  264,  265. 
Breezes,  land  and  sea,  262. 
Brocken  specter,  174. 
Bull's-eye  squalls,  261. 
Buran,  264. 


Calms,  equatorial,  208. 
Carbon  dioxide,  7,  8,  12. 
Centrifugal  force,  181,  201,  202. 
Centigrade,  32,  33. 
Central  America,  climate  of,  310. 
Chinook  hot  winds,  266. 
Circulation   of  air    (see   under  Atmos- 
pheric). 

Cirrus  clouds,  130. 
Girro-cumulus  clouds,  130, 131. 
Cirro-stratus  clouds,  130,  131. 
Climate.  269,  293-312. 

continental,  296. 

denned,  269,  293. 

effect  of  altitude  on,  301. 


Climate,  effect  of  forests  on,  299-301. 

effect  of  mountain  ranges  on,  298. 

effect  of  oceanic  circulation  on,  298. 

effect  of  wind  on,  297,  298. 

land  or  continental,  296. 

oceanic,  296. 

of  Africa,  307. 

of  Asia,  308. 

of  Central  America,  310. 

of  Europe,  307. 

of  North  America,  311. 

of  South  America,  310. 

of  United  States  (see  under  United 
States),  313-363. 

polar,  303. 

solar,  295. 

telluric  or  physical,  295. 

temperate,  302. 

tropical,  302. 

water  or  oceanic,  296. 
Climatic    subdivisions    of    the    United 

States,  317. 
Climatic  zones,  301. 
Climatological  elements,  293,  294. 

geographical   distribution   in  United 

States,  321-361 
Climatology,  293. 
Cloud-bursts,  261. 
Cloudiness,  annual  change  of,  138. 

degree  of.  137. 

diurnal  change  of,  137. 

in  United  States,  352-354. 

variation  of,  with  latitude,  138, 
Clouds,  129-139. 

amount  of,  137-139. 

dissipation  of,  134. 

formation  of,  129,  134-137. 

forms  or  kinds  of,  130-134, 

in  a  thunderstorm,  2580 

luminous,  13. 
Cold-wave  frosts,  290. 
Cold  waves,  287. 

in  United  States,  332. 
Cold  winds,  264. 
Colors  of  the  sky,  170. 
Condensation,  119. 

causes  of,  143. 

Conditions,  meteorological,  17, 
Conduction,  19. 
Conductors  of  heat,  19,  2Oo 


INDEX. 


367 


Conservation  of  areas,  182. 
Contraction  and  expansion  of  air,  33. 
Continental  or  land  climate,  296. 
Convection,  20. 
Corona,  171. 

Cumulus  clouds,  130,  131. 
Cumulo-cirrus  clouds,  130. 
Cumulo-nimbus  clouds,  130,  131. 
Currents,  air  (see  Air),  101. 
Cyclones,  214-234,  235,  238-240. 

calm  at  center  of,  223. 

classes  of,  216. 

coalescence  of,  227. 

effect  of  friction  on  winds  in,  233. 

formation  of,  231-234. 

frequency  of,  227-229,  316. 

in  United  States,  314-317. 

irregular  movement  of,  282-285. 

locating  center  of,  274. 

magnitude  of,  216,  223. 

moisture  in,  220. 

of  temperate  and  colder  zones,  216. 

of  torrid  zone,  216,  217. 

paths  of,  224-229,  283,  314-316. 

permanent,  240. 

pressure  of  air  in,  217-219. 

rainfall  in,  221,  273. 

region  of  maximum  number  of,  227. 

relation  to  anticyclones,  235,  238-240. 

rotation  of,  214-216,  233. 

secondary,  231. 

separation  of,  227. 

shifting  of  winds  in  passage  of,  275. 

temperature  in,  219,  220. 

velocity  of,  225. 

weather  conditions  of,  273. 

winds  in,  222,  233. 
Cyclonic  motion,  source  of,  232. 


Days,  length  of,  24,  25. 

Deflecting    force    of    earth's     rotation, 

195-199. 

Density  of  air,  75,  234. 
Dew,  162. 
Dew-point,  123. 
Diathermancy,  20. 
Diffusion  of  heat,  19. 
Diffraction  of  light,  169. 


Direction  of  wind,  103. 

Doldrums,  208. 

Droughts  in  United  States,  348. 

Dry  stage,  120. 

Dust  particles,  9,  169. 

whirlwinds,  261. 
Dynamical  meteorology,  18. 


Earth,  inclination  of  axis  of,  22. 

revolution  of,  21. 

rotation    of    (see    under    Rotation), 

22. 

Eddy  winds,  103,  231. 
Ecliptic,  plane  of  the,  22. 
Elastic  force  of  a  gas,  n. 
Electricity,  atmospheric,  175-179. 

periodic  changes  in,  178. 
Elements,  meteorological,  16. 
Energy  of  the  air,  182. 
Equilibrium  of  air,  185-187. 

indifferent,  185,  186. 

stable,  185.  186. 

unstable,  186. 
Europe,  climate  of,  307. 
Evaporation,  118,163-165. 

annual  amount  of,  165. 

from  the  ground,  165. 

measurement  of,  124,  164. 

periodic  variations  in,  165. 
Evaporimeter,  164. 
Expansion  and  contraction  of  air,  33. 
Expansive  force  of  a  gas,  n. 
Eye  of  the  storm,  230. 


Fahrenheit,  32,  33. 

Fiducial  points,  32. 

Floods,  prediction  of,  290. 

Foehn  process,  hot  waves  due  to,  288. 

rapid  change  of  temperature  due  to 
267. 

winds,  265. 
Fog  image,  174. 
Fogs,  120,  134-140. 

formation  of,  134-137 

ground  fog,  134. 

ocean,  140. 


368 


ELEMENTARY   METEOROLOGY. 


Force,  gradient,  100. 

of  wind,  101,  105. 
Forests,  air  temperatures  in,  299. 

effect  on  climate,  299-301. 

humidity  in,  300. 
Fracto-cumulus  clouds,  134. 
Fracto-nimbus  clouds,  134. 
Friction,  of  air  layers,  in. 

effects  of,  on  air  motions  in  cyclones, 

233- 
decrease  of  wind  velocities  through, 

112.     ' 

Frosts,  cold-wave,  290. 

dates  of  earliest  and  latest,  in  United 
States,  333-335- 

night,  162,  291. 

prediction  of,  290. 
Frozen  earth,  region  of,  71. 
Fungi,  9. 


Gases,  arrangement  in  air,  12. 

density  and  volume  of,  10. 

elasticity  of,  n. 

pressure  of,  10. 
Glory,  or  fog  image,  174. 
Glow  of  sky,  170. 
Gradient,  barometric,  zoo,  201,  202. 

force,  loo. 
Ground,  evaporation  from,  165. 

temperature  of,  70,  299. 


H 

Hail,  159. 

fall  in  a  thunderstorm,  259. 

stage,  121. 
Hailstorms,  259. 
Halo,  171-174. 
Halos,  classes  of,  172. 

explanation  of,  173. 
Heat  (see  also  Solar,  and  Temperature). 

absorbed  by  air,  27-29,  183. 

absorbed  by  land  and  water,  29,  30, 
183,  296. 

conductors  of,  19,  20. 

denned,  19. 

diffusion  of,  19. 

in  the  air,  20, 182. 


Heat  of  the  earth   retained  by  atmos- 
phere, 29. 

radiation  of,  from  air,  183. 

rays,  absorption  of,  by  air,  28. 

reflected,  20. 

solar,  20-31. 

terrestrial,  21. 

unit  of,  27. 

zones  of  the  earth,  306. 
Heating  of  air  in  descending  currents, 

184. 

"  High,"  278. 

Horizontal  air  currents,  101. 
Hot  waves,  causes  of,  288. 

prediction  of,  288. 
Hot  winds,  264,  265. 
Humidity,  122-129. 

absolute,  122. 

absolute,  in  United  States,  348. 

annual  march  of  absolute,  127. 

annual  march  of  relative,  126. 

diurnal  change  in  absolute,  127. 

diurnal  change  in  relative,  125. 

geographical  distribution  of,  126. 

in  forests,  300. 

in  United  States,  348-352. 

measurement  of,  123,  124. 

relative,  122,  126,  127. 

relative,  in  United  States,  350. 
Hurricanes,  216,  229. 

predictions  of,  289. 
Hygrometer,  hair,  124. 
Hygrometry  of  the  air,  118. 

I 

Inclination  of  earth's  axis,  22. 

Indian  ocean,  monsoon  winds  of,  209- 

212. 

Indifferent  equilibrium,  185. 
Is-abnormals,  58-61. 
Isobar,  85. 
Isobaric  charts,  88. 

of  the  world,  88-94. 

of  United  States,  359,  361. 
Isobaric  surfaces,  85-88. 

graphical  representation  of,  86. 

inclination  of,  85. 
Isotherms,  51,  278. 

course  of  annual,  53. 


INDEX. 


369 


Isothermal  charts,  51. 

of  the  world,  52-57. 

of  United  States,  322-324. 
Isothermal  surface,  51. 


Kahnisin,  265. 


K 


Land  and  sea  breezes,  102,  262. 

Land  or  continental  climate,  296. 

Land  surface,  action  of  heat  on  a,  29,  30. 

temperature  of,  51,  296. 
Latitude,  15. 
Leste,  265. 
Leveche,  265. 
Light,  diffraction  of,  169. 

rays  absorbed  by  air,  28. 

reflection  of,  168. 

refraction  of,  166. 
Lightning,  176-178. 

forms  of,  177. 

tornadoes  accompanied  by,  254. 
Longitude,  15,  16. 
"  Low,"  278. 
Luminous  phenomena,  166-179. 


M 

Maps,  weather,  271,  278-282. 
Mercury,  31. 

Meridians,  effect  of  convergence  of,  193. 
Meteorology,  defined,  7. 

dynamical,  18. 

statical,  18. 

Meteorological  conditions,  17. 
Meteorological  elements,  16. 

distribution  of,  18. 
Meteorological  instruments,  17. 
Microbes,  9. 
Mirage,  169. 
Mistral,  264. 
Mock  suns,  172. 
Moist  air,  ascending  currents  of,  184. 

descending  currents  of,  184. 
Moisture  of  the  air,  118-142. 

amount  of,  122-128,  140. 

as  cloud  and  fog,  129-140. 


Moisture  of  the  air  as  precipitation,  142- 
165. 

as  vapor,  122-129. 

in  anticyclones,  236. 

in  cyclones,  220. 

in  United  States,  348-354. 

measurement  of,  123,  124. 

on  weather  maps,  279. 

relation  to  temperature,  122. 

stages  of,  120. 
Molds,  9. 
Monsoon  winds,  102,  116. 

of  Indian  Ocean,  209-212. 
Mountain  and  valley  breeze,  102, 262. 
Mountain  ranges,  effect  on  climate,  298. 

effect  on  rainfall,  146. 


N 

Neutral  planes,  193,  194. 
Nights,  length  of,  24,  25. 
Nimbus  clouds,  130,  131. 
Nitrogen,  7,  8,  12. 
North  America,  climate  of,  311. 
Norther,  black,  265. 
Northers,  264. 


Ocean,  temperature  of,  71. 

Oceanic  climate,  296. 

Oceanic  circulation.climatic  effects  of,  298. 

Optics,  atmospheric,  166-175. 

Oxygen,  7,  8,  12. 


Pampero,  264. 
Periodic  change,  17. 
Physical  or  telluric  climate,  295. 
Plane  of  the  ecliptic,  22. 
Polar  climate,  303. 

Precipitation  (see  also  under  Rain,  Rain- 
fall, Snow,  and  Snowfall),  142-162. 
average  annual    for    United    States, 

335-339- 

frequency  in  United  States,  346. 
Prediction  of  cold  waves,  287. 
of  frosts,  290. 
of  hurricanes,  289. 


370 


ELEMENTARY   METEOROLOGY. 


Prediction  of  river  floods,  290. 

of  thunderstorms,  286. 

of  weather  (see  also  Weather  predic- 
tions), 271. 
Pressure  of  the  air,  73-100. 

at    various    altitudes,    73-75,    77-80, 
96. 

at  various  latitudes,  95,  96. 

annual  change  of,  82. 

areas  of  high,  88. 

areas  of  low,  88. 

classified  distribution  of,  99. 

diurnal  change  of,  81. 

effect  of  air  motions  on,  201. 

excessive,  in  anticyclones,  238. 

geographical  distribution,  88-95. 

gradients  (barometric),  ioo,  201,  202. 
.  graphical  representation  of,  86. 

high,  88,  99. 

hourly,  81. 

in  United  States,  359,  361. 

in  anticyclones,  236,  238. 

in  cyclones,  217-219. 

in  tornadoes,  247. 

irregular  oscillations  of,  83. 

latitude  of  maximum,  203. 

long-period  oscillations,  97. 

low,  88,  99. 

monthly,  83. 

monthly  oscillations,  84. 

observations  of,  80. 

oscillations  of,  in  winter  and  summer, 
84,  238. 

physiological  effects  of,  97. 

variability  of,  97. 
Pressure  of  wind,  101,  105. 
Psychrometer,  124. 


R 

Radiation,  20. 

of  heat  from  air,  183. 

solar  (see  Solar) ,  21-29. 
Rain  (see  also  Rainfall),  142. 

agricultural,  in  United  States,  341. 

artificial  production  of,  159. 

causes  of,  142-145. 

probability  of,  157. 

stage,  120. 
Rainbows,  174. 


Rainfall  (see  also  Rain),  142-162. 

amount  of,  145. 

annual,  146. 

average,  in  United  States,  335-339. 

diurnal  change  of,  145,  146. 

duration  of,  156. 

effect  of  mountains  on,  146. 

excessive,  in  United  States,  342. 

frequency  of,  in  United  States,  346 

gauge,  145. 

geographical  distribution  of,  148. 

greatest  daily,  146. 

in  Africa,  153,  156. 

in  Arctic  regions,  156. 

in  Australia,  156. 

in  cyclones,  221,  273. 

in  Europe,  155. 

in  North  America,  155. 

in  South  America,  155. 

in  Siberia,  155. 

in  the  Doldrums,  152. 

in  United  States,  335-348. 

intensity  of,  156. 

long-period  fluctuations  of,  157,  158, 

probability  of,  157. 

seasonal  distribution  of,  150. 

shifting  of  lines  of  equal,  158. 

subtropical,  153,  154. 

temperate,  154-156. 

tropical,  152. 

types  in  United  States,  340. 

variability  of,  156. 

variation  toward  interior  of  continents, 
146. 

variation  with  altitude,  147. 

variation  with  latitude,  147. 

zones,  150. 
Reaumur,  33. 
Reflection  of  light,  168. 

from  dust  particles,  169. 
Refraction  of  light,  166. 
Revolution  of  the  earth,  21. 
River  floods,  prediction  of,  290. 
Rotary    motion,   in    anticyclones,    214- 
216. 

in  cyclones,  214-216,  233. 

in  tornadoes,  245. 
Rotation  of  the  earth,  22. 

deflecting  force  of,  191,  195-199. 

influence  of,  on  air  currents,  195. 


INDEX. 


371 


St.  Elmo's  fire,  178. 

Saturation,  118,  119,  122. 

Sea  and  land  breeze,  102,  262. 

Sea  level,  barometric  pressures  at,  77. 

Seasons,  40-42. 

Signals,  storm,  291. 

Sirocco,  265. 

Sky,  aspect  of,  on  weather  maps,  279. 

colors  of,  170. 

glow  of,  170. 
Snow,  159. 

crystals,  160. 

density  of,  161. 

eaters,  267. 

line,  67,  69,  70. 

stage,  121. 
Snowfall,  145,  159-162. 

in  United  States,  343-346. 

latitudinal  limit  of,  159. 

measurement  of,  161. 

relation  to  temperature,  121. 
Soil,  effect  of  forests  on  temperature  of, 

299. 

Solar  climate,  295. 
Solar  constant,  27. 
Solar  heat,  20,  21. 

absolute  quantity  of,  27. 

absorbed  by  atmosphere,  27-29,  183. 

absorbed    by    land    and    water,    29, 

3°- 
at  earth's  surface  without  atmosphere, 

25- 
Solar  radiation,  21. 

on  earth's  surface,  22-25,  27,  28. 

on    northern    and    southern    hemi- 
spheres, 29. 

Solar  rays  refracted  by  air,  167,  168. 
South  America,  climate  of,  310. 
Spouts,  103,  259,  260. 
Squalls,  103. 

bull's-eye,  261. 

white,  261. 

Stable  equilibrium,  185. 
Statical  meteorology,  18. 
Stratus  clouds,  130,  134. 
Strato-cirrus  (alto-stratus)  clouds,  130, 

131. 
Strato-cumulus  clouds,  130,  131. 


Storm    (see    Tornado,    Thunderstorm, 

Squalls). 
Storm,  eye  of  a,  230. 

signals,  291. 

warnings  and  weather  charts,  276. 
Straight  blows  (derechos),  241. 
Sun  (see  also  Solar). 

apparent  motion  of,  25. 

distance  of  earth  from,  22, 

visibility  of,  24. 
Sun  dogs,  172. 


Telluric  or  physical  climate,  295. 
Temperate  climate,  302. 
Temperature,  19-72. 

abnormal  average,  58-61. 

annual  change  of,  42-44. 

anomaly,  47. 

areas  of  high  or  low,  46. 

average,  for  year,  45,  46. 

below  freezing,  in  United  States,  329. 

change  with  altitude,  48-50,  185. 

diurnal  change  of,  35-39. 

effect  of  forests  on,  299. 

effect  on  air  motions,  205,  232. 

extremes,  annual,  43,  62-66. 

extremes,  diurnal,  36,  37. 

extremes  in  United  States,  325. 

fluctuations  of,  34,  35. 

fluctuations  in  United  States,  328. 

geographical  distribution  of,  50. 

hourly,  39. 

in  anticyclones,  237. 

in  cyclones,  219,  232. 

in  United  States,  322-335. 

irregular  changes  of,  45. 

limit  of  freezing,  65. 

monthly,  44,  45. 

normal,  54. 

observations  of,  34. 

of  forests,  299. 

of  ground,  70. 

of  land  and  water  surfaces,  51,  296, 

of  ocean,  71. 

of  small  bodies  of  water,  72. 

of  the  hemispheres,  58. 

on  weather  maps,  278. 

range  of,  45,  60-63. 


372 


ELEMENTARY   METEOROLOGY. 


Temperature,  relation  to  moisture,  122. 

seasonal  fluctuations  of,  35. 

variability  of,  47. 

variability  in  United  States,  330. 

zones  of  the  earth,  304-306. 
Terrestrial  heat,  21. 
Thermal  absorption,  20. 
Thermal  days,  26,  27. 
Thermometer,  19,  31-33. 

Centigrade,  32,  33. 

Fahrenheit,  32,  33. 

graduation  of,  32. 

mercurial,  31,  33. 

principle  of  construction,  31. 

Reaumur,  33. 

scales,  31. 
Thunder,  176,  250. 
Thunderstorms,  241,  249-259. 

attendant  phenomena,  249-251. 

changes  in   meteorological   elements 
during,  255. 

classes  of,  251. 

clouds  in,  250,  258. 

cyclonic  or  progressive,  251,  253. 

growth  of,  254. 

hail  fall  in,  259. 

heat  or  stationary,  251,  252. 

ideal  sketch  of  a  thunderstorm,  257. 

in  United  States,  361-363. 

maintenance  of,  253,  254. 

prediction  of,  286. 

progressive  movement  of,  253,  254. 

rain  in,  250,  256,  257. 

region  of  greatest  frequency  in  United 
States,  363. 

time  of  occurrence  of,  255. 

variation  of  number  of,  with  latitude, 

255- 

velocity  of,  255. 
wind  squall  in,  253. 
winter,  251. 

Tornadoes,  103,  216,  241-249,  259. 
air  motions  in,  243-245. 
destructive  winds  in,  247. 
formation  of,  243. 
frequency  of,  249. 
lightning  in,  254. 
low  pressure  at  center  of,  247. 
moisture  in,  245. 
multiple,  249. 


Tornadoes,  paths  of,  246. 

polar  limits  of,  246. 

prediction  of,  286. 

pressure  of  air  in,  247. 

regions  of  greatest  frequency  of,  246. 

rotation  of,  245. 

safety  in,  249. 

time  of  occurrence  of,  245. 

velocity  of,  247. 

vertical  air  motion  in,  244,  248. 

wind  force  and  velocity  in,  245,  248. 
Trade  winds,  208. 
Transparency  of  the  air,  166. 
Transpiration,  163. 
Trees,  temperature  of,  299,  300. 
Tropical  climate,  302. 
Twilight,  length  of,  24,  25. 
Typhoons,  216,  229,  230,  290. 


U 

Unstable  equilibrium,  186. 

United  States,  anticyclones  in,  314-317 

climate  of,  313-363. 

climate  of  Atlantic  slope,  321. 

climate  of  central  prairies,  319. 

climate  of  Great  Plains,  319. 

climate  of  Pacific  coast,  318. 

climate  of  plateau  region,  318. 

climate     of     western     Appalachian 
slope,  320. 

climatic  types  in,  313. 

cloudiness  in,  352-354. 

cold  waves  in,  332. 

cyclones  in,  314-317. 

destructive  storms  in,  361. 

droughts  in,  348. 

earliest  and  latest  frosts  in,  333. 

humidity  in,  348-352. 

month   of  maximum   and   minimum 
wind  in,  356. 

rain-bearing  winds  in,  361. 

rainfall  in,  335-348. 

rainfall,  types  in,  340. 

snowfall  in,  343-346. 

temperature  of,  322-335. 

tornadoes  in,  246. 

weather  maps,  279-282. 

wind  direction  in,  358-361. 

wind  velocity,  355-358. 


INDEX. 


373 


Valley  and  mountain  breeze,   102,   262. 

Vane,  wind,  104. 

Vapor  in  air  (see  Humidity),  7,  118, 122. 

amount  of,  128,  140. 
Vegetation,  climatic  effect  of,  299. 
Vertical  air  movements,  102,  200. 
Volcanic  winds,  103. 


W 

Water  (see  also  Moisture). 

evaporation  of,  118,  163-165. 

or  oceanic  climate,  296. 

surface,  heat  received  by,  29,  30. 

surface,  temperature  of,  51,  296. 

temperature  of  small  bodies  of,  72. 

vapor  (see  Humidity), 7, 118, 122-129. 
Waterspouts,  241,  259,  260. 
Weather,  269,  293. 

and  weather  predictions,  269-292. 
Weather  charts,  271,  278,  280-282. 

air  pressure  on,  278. 

aspect  of  sky  on,  279. 

construction  of,  278. 

introduction  of,  276. 

moisture  on,  279. 

temperature  on,  278. 

use  of,  for  weather  predictions,  276. 

variations  in,  284. 

wind  on,  279. 

Weather  conditions,  absolute  and  rela- 
tive, 269. 

current,  271. 

in  a  cyclone  and  an  anticyclone,  272. 
Weather  maps  (see  Weather  charts). 
Weather  predictions,  271. 

accuracy  of,  292. 

distribution  of,  291. 

for  different  regions,  284. 

long-range,  292. 

methods  of  making,  274-285. 

scope  of,  282. 

use  of  storm  charts  for,  276. 
Weather,  seasonal,  270. 
Weight  of  air  (see  Pressure) ,  10,  13. 
Whirls,  movement  of  air  in,  213. 
Whirlwinds,  103. 

dust,  261. 


White  squalls,  201. 

Wind  direction,  103-106,  207. 

annual  change  of,  116. 

daily  change  of,  113. 

in  United  States,  358-361. 

locating  cyclone  center  by,  274. 

observation  of,  105,  207. 

resultant  or  normal,  106,  116. 
Winds  (see  also  Air  motions),  101-117. 

avalanche,  267. 

classification  of,  102. 

cold,  264. 

eddy,  103,  231. 

effects  on  climate,  297. 

foehn,  265. 

force  of,  loi,  105. 

hot,  264,  265. 

hurricanes,  216,  229. 

in  anticyclones,  237. 

in  cyclones,  222. 

in  tornadoes,  245,  247. 

in  United  States,  355-361. 

land  and  sea  breezes,  262. 

local  and  miscellaneous,  241-268. 

monsoon,  102,  116,  209-212. 

mountain  and  valley,  262. 

of  Atlantic  Ocean,  113. 

of  Indian  Ocean,  209-212. 

on  weather  charts,  279. 

periodic  local,  262. 

shifting  in  passage  of  cyclone,  275. 

trade,  208. 

volcanic,  103. 
Wind  velocity,  101,  104,  106-113. 

annual  change  of,  109,  no. 

average  annual,  no. 

average  monthly,  no. 

change  with  altitude,  107,  in. 

change  with  latitude,  112. 

diurnal  change  of,  106-109. 

in  United  States,  355-358. 

in  winter,  205. 

measurement  of,  104. 

retardation  of,  in,  199. 

theoretical  east  and  west,  205. 


Zondas,  265. 
Zones,  301. 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
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WILL.  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  Sl.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


SEP    27  t! 


FEB  Q 


NOV   6    1942 


RETURNED  TO 
MATH.-STAT.  LIB 


1959 


24W60MJ 


j&ttlO 

ll]an'62KLX 


1840 


LD 

DEC  2 11961 
JUL  03  1974  21 


WAR   1»'W" 


LI)  LM 


